I have finally gotten around to compiling my own list, so here it is.
Some versions of rage are more defensive than others, though none of them are particularly mystical or provide non-attack-based interactions with challenges.
The baseline: +Str, +Con, +Will, −AC. Two things we like, one we don’t care about, and one negative but it’s really small. Even alone, we like it on more than off.
Ferocity (“Urban Class Features,” Cityscape web enhancement)
Barbarian-only alternative class feature, though many DMs are amenable to letting other rage classes get it.
+Dex instead of +Con; a net loss, but not a terrible one. The real story is activation: immediate action, even out of your turn, even if surprised, which can go a long way towards ensuring you have your various rage benefits when you need them.
Feats are a big consideration here: Frantic Rage and Instantaneous Rage give ferocity’s top selling points to rage. But that’s two feats, and Frantic Rage in particular has its own complications (see below).
Whirling Frenzy (Unearthed Arcana)
A rage variant, availble to any class that gets rage.
Almost purely about attacking, and thus not relevant here. The tiny dodge bonus to AC isn’t worth much at all.
Mountain Rage (Races of Stone)
Goliath barbarian racial substitution level only, and not to any other race or class. DMs are unlikely to go for other combinations. Stacks on top of rage, which could arguably be some other rage variant.
Makes you Large and stronger, but without attacking, almost nothing about that is valuable to us—certainly not when it requires LA +1.
Resilient Rage (Dragon vol. 330)
Implacable barbarian, a variant class, exclusive. Harder to separate from that class.
+Dex and +Con, which is ideal, and also a tiny amount of DR/–. Implacable barbarians cannot take the lion spiritual totem, which usually makes this a non-starter, but that’s a non-issue for us. With Instantaneous Rage, this is absolutely the best version of rage for us.
Berserker Strength (Player’s Handbook II)
An alternative class feature that can be taken at “level: 1st” that replaces “rage (or any later improvements to that class feature).” The description discusses barbarians, and the feature is listed in the Barbarian section of the Expanded Classes chapter, but RAW nothing explicitly says you have to be barbarian to use it—any class that gets rage at 1st can instead get berserker strength.
+Str, +all saves, DR/–, −AC. No +Con, but all saves is quite nice, and the DR/– is better than resilient rage (by 1, so still tiny).
And berserker strength explicitly notes that “Any effects that normally only apply during your rage applies whenever your berserker strength is active.”
The tricky thing about berserker strength is that it doesn’t have uses per day at all: it is simply on when your hp is below a 5 × your class level, and off when healed above that. This is, generally speaking, a massive drawback: consider how highly we regarded ferocity for its ability to activate whenever you want, and how Instantaneous Rage was a given for other forms of rage. Not an option here, and when we’re trying to stack our defenses onto our rage, that could well be suicidal. However, see “rage-cycling” at the end of this answer for one work-around.
These are ordered with base classes first, in alphabetical order, then prestige classes, also in alphabetical order.
Barbarian (Player’s Handbook)
Obviously, this is the typical source of rage. Also some damage reduction, but it’s such a tiny amount that it’s not (remotely) worth the levels. Another use of rage every 4 levels is really mediocre, and waiting until 11th for greater rage is just a no-go. Barbarian 2nd, for uncanny dodge (or something you trade uncanny dodge for), is probably as far as you ever want to go with the class.
True barbarian is the only way to access ferocity, mountain rage, or resilient rage.
Druidic Avenger (Player’s Handbook + Unearthed Arcana)
The druidic avenger variant druid gets rage, and is far more appropriate to this goal than the barbarian is in the first place. Rage progression is the even worse than barbarian, but obviously druid is offering a whole lot more in terms of other class features.
Stacking between barbarian and druidic avenger is unclear. They’re probably separate progressions (so a 1st-level barbarian/1st-level druidic avenger gets two daily rages), but it’s not immediately clear that this is the case; Unearthed Arcana doesn’t really address it. See this Q&A for more details.
Druidic avenger is also notably compatible with this variant druid, also from Unearthed Arcana, which trades wild shape for a lot of monk and ranger features—a very good deal if you were never going to take enough levels to get wild shape anyway (an utterly atrocious deal if you were going to get wild shape, of course).
Half-orc Paragon (Unearthed Arcana)
Half-orc paragon gets a daily usage of rage at 2nd level. Notable mostly for its lack of alignment requirements, though of course the druidic avenger’s requirements are quite loose (just any neutral).
Since half-orc is a terrible race, this probably isn’t too great an idea. The first level is pretty much dead, too.
Bear Warrior (Complete Warrior)
Turn into a bear when you rage. Bears are big and tough, so there’s some defensive benefit there, but really it’s about being a bear and mauling people. The type of bear improves at 5th and 10th, and there’s an extra rage per day at 7th, but the class has a lot of dead levels.
Chameleon (Races of Destiny)
At 3rd, 6th, and 9th level, get a daily use of mimic class ability, which includes rage as an option. Chameleon is a cool class, but this is a really slow way to get more rage, and it provides no other benefits. And since its best features involve spellcasting, it’s hard to use with rage.
Champion of Gwynharwyf (Book of Exalted Deeds)
Champion of Gwynharwyf stacks with barbarian levels to determine how and how often you can rage, so that’s something. More importantly, at 2nd level you get the ability to cast champion spells while raging, and at 4th you get a rather-potent Intimidate aura whenever you rage. And this is on top of a class that is already very defensively powerful, regardless of rage: there’s DR, resistances, immunities, and divine grace in there.
Unfortunately, it requires that you take two feats and absolutely set them on fire, and the BAB +6 requirement is pretty rough too. And the spellcasting you get and can cast during a rage is rather mediocre.
Frenzied Berserker (Complete Warrior)
Note that frenzied berserker is a very divisive class. There are ways for a party to protect themselves against the risks of associating with a frenzied berserker, but a lot of groups don’t want the bother. Always ask before playing one—ask everyone, not just the DM, because it’s your fellow players who are going to have to deal with it the most. (While you’re at it, ask about the Righteous Wrath feat, as discussed at the end of this answer—even if your group is OK with the problems, if you can fix those with just one feat and you don’t, that’s pretty rude.)
Despite frenzy being a very-intentionally “dangerous” ability (if you run out of enemies you start attacking allies), frenzied berserker provides an incredible defensive benefit at 4th level, deathless frenzy. That allows you to literally ignore dying until your frenzy finishes.
Frenzy also applies all your rage benefits, so the daily usages of frenzy effectively count as more daily uses of rage, and you get one every odd level, which is good. Your daily uses of non-frenzy rage don’t let you frenzy, however.
This class is actually better than I thought: it takes three feats, which is a lot, but two of them (Frozen Berserker, Power Attack) are strong (note Tome of Battle’s Stone Power offers a defensive alternative to Power Attack), and the last (Improved Unarmed Strike) is a common bonus feat, available through several of the classes we were looking at anyway (city-brawler barbarian, that monk+ranger variant druid). Frostrage is mediocre (+4 natural armor, +6 at 4th level, plus some stuff for unarmed strikes), but freezing blood and absorb cold are both quite decent. The big problem is that this is five levels of no extra uses of rage, and while freezing blood and absorb cold are quite decent, that’s just “decent.”
Hida Defender (Oriental Adventures)
Gets a use of rage on every even level, which is quite a pace compared to the barbarian. Gets DR on most of the odd levels, too, for up to DR 4/–: far too little to really make much difference, but again, way faster than the barbarian gets it. Requirements aren’t too onerous, either (heavy armor proficiency, Power Attack and Weapon Focus). Not really a great choice but it exists.
Kinslayer (Drow of the Underdark)
Little of note, but there is a use of rage at 1st, 5th, and 9th. Mettle (not tied to rage) at 1st isn’t bad either. But not worth four terrible feats, even if you can do it at really low level.
Rage Mage (Complete Warrior)
This is the obvious choice for this concept, but jeez is it a weak class. Spell rage is separate from your regular rage and you only get a daily use of it at 1st, 5th, and 10th, so you’re really limited in using it. It only improves spellcasting on even levels, so the spells you can finally use are really terrible. It’s arcane-only, so druidic avenger can’t qualify and can’t be progressed by it (though if you get in some other way, druid spells can benefit from spell rage, angry spell, and spell fury).
Overall, this probably isn’t worth it.
Runescarred Berserker (Unapproachable East)
This class oozes cool, and literally everything about it is on-theme, but its spellcasting is even worse than the rage mage, and the prerequisites are harsher too.
Singh Rager (Oriental Adventures)
So singh rager gets “lion’s fury” which is literally a copy-paste of rage—unclear if that means it counts as rage for the sake of everything else we have improving rage. But it gets three uses at once, which is a pretty big deal. Problem: there’s an error here, and the feature says both 2nd level and 3rd level:
Lion’s Fury (Ex): At 2nd level, [...] A singh rager can enter a rage three times per day at 3rd level,
Note that prior to the line about getting three uses at 3rd level, there is nothing in the feature about how often it can be used. One could even argue that, strict RAW, you get infinite uses at 2nd level, since at that level no rule applies that limits how much it can be used. For what it’s worth, the table lists “Lion’s fury 3/day” at 2nd.
You definitely get another use at 6th and again at 10th, not that the class has much of anything else worth hanging around that long for. Requirements aren’t too terrible, though the Lawful requirement could something of an issue.
Totem Rager (Magic of Incarnum)
Probably the best option for dedicated “rage plus X” classes, meldshaping plays nice with rage and the the progression here is fairly strong (meldshaping on all levels but 1st), though the chakra progression is mediocre. The big problem is that soulmelds, particularly totemist souldmelds, are very attack-oriented. You might be spending feats on Shape Soulmeld to poach some incarnate soulmelds. On top of that, the actual rage progression is meh: an extra usage at 2nd and 8th, and that’s it.
These are listed in descending order of my assessment of their value, that is, the best feats are on top.
Instantaneous Rage (Complete Warrior)
Especially if trying to focus on defensive benefits of rage, this feat is incredible: it allows you to rage any time you like, regardless of whether it’s your turn, whether you are flat-footed or surprised, or whatever else. It doesn’t even require an immediate action (which matters more for immediate actions’ prohibition on use while flat-footed than it does for the swift action lost from your next turn). As long as you are aware of a threat and able to act at all, you can turn on rage.
Really, the only reason not to take this feat is if you have ferocity—which has the benefit largely built in—or berserker strength—which isn’t something you can activate manually in the first place.
Blazing Berserker (Sandstorm)
Gain the fire subtype during a rage, gaining immunity to fire damage but vulnerability to cold damage. Awful on its own, but amazing in combination with Frozen Berserker.
Frozen Berserker (Frostburn)
Gain the cold subtype during a rage, gaining immunity to cold damage but vulnerability to fire damage. With Blazing Berserker, you are immune to both fire and cold damage, and therefore your vulnerability to those damage types is irrelevant (150% of 0 is still 0).
Blazing+Frozen Berserker is probably the best defense you can get with rage. The only competition is...
Mad Foam Rager (Player’s Handbook II)
Once per rage,
As an immediate action, you can choose to delay the effect of a single attack, spell, or ability used against you.
This is flat-out incredible. A round is an incredibly long respite for something that could be truly awful, and often plenty of time to apply protections against it (can your cleric ally cast protection from evil on you before that dominate person lands?).
Extra Rage (Complete Warrior)
Extra Rage gives two uses of rage for a feat, which is an absolute bargain compared to the several levels you need to take to get even one usage from most classes. You can take it as much as you want, too, to keep getting more rage. Typical combat day has four encounters, so maybe you’re good at four rage per day, but you could get more.
Raging Luck (Eberron Campaign Setting)
Get a temporary action point during a rage, which is amazing for a whole lot of potential things. Action points are strong and usually very limited, so this is a great feat. There are also tons and tons of feats that build off of action points, allowing you to turn rage into serious fuel for powerful options, including a lot of rather “mystical” stuff.
Ettercap Berserker (Unapproachable East)
When raging, you gain a +6 bonus to Constitution.
RAW, this stacks with the +4 bonus to Constitution that is rage’s default. They presumably meant for it to replace that bonus, but that isn’t what they said.
If the DM doesn’t buy that, though, the feat is still very worthwhile for rage variants that don’t get Constitution bonuses to begin with, like ferocity, whirling frenzy, or berserker strength. No worries about stacking in that case!
Rage Casting (Dragon vol. 310)
With thanks to HeyICanChan’s answer for pointing it out.
The requirements here are somewhat unpleasant—Combat Casting, which is garbage for anyone and particularly for someone with a colossal Constitution like you should have, and Quicken Spell, which is some very expensive metamagic that will be hard to use. But at least they aren’t forcing you to really divert yourself off into other classes or whatever—neither feat has any requirements, so you can just take them.
The feat does two things:
You can use free-action spells while raging. Note that swift and immediate actions were not part of the original rules, and Quicken Spell and things like feather fall used to use free actions. That’s what the feat is likely referring to, and so swift- and immediate-action spells should be allowed under this feat, rather than just free-action spells (which I don’t believe even exist anymore). You’ll have to double-check with the DM, though.
You can activate command-word, spell-completion, and spell-trigger magic items while raging. No reference is made in this sentence to the activation time of those items (and really, absent Rules Compendium, the rule was that activating those items was always a standard action regardless of the underlying spell anyway). So no ruling should be necessary on this part.
This is a potentially big deal, because magic items open up a whole lot of options for us. And see “rage-cycling” below for really playing this up to the hilt.
Intimidating Rage (Complete Warrior)
Free-action demoralize against a single target while raging once per encounter. It’s a shame it’s a free action (instead of something that just happens when you rage), so you can’t use it out-of-turn with Instantaneous Rage, and it specifies a single target, so Complete Scoundrel’s Never Outnumbered doesn’t apply, but still, grab some fearsome armor and this is still pretty potent. A fighter has to wait until 9th level to get swift-action demoralize, so in that competition this is a big win.
Frantic Rage (Faiths of Eberron)
The benefit of this feat is great: swap the Strength bonus of rage to Dexterity. Dexterity is pretty important to us, and Strength is worthless, so that’s great.
The big problem is you need access to the Madness domain in order to take it. That probably means a cleric dip, which is generally fine as cleric dips are awesome, but the Madness domain is crap and that strongly devalues the cleric dip—and then you need to burn a feat on this on top of that.
Only worth considering those hoops if a druidic avenger or something. If you’re an actual barbarian, just take ferocity or resilient rage, and skip this.
Shared Fury (Races of the Wild)
It’s awkward to get because none of the classes we’re interested in gets an animal companion, and on top of that, RAW, it’s not clear that any of our feats boosting rage would be transferred. If yes, it’s a strong feat that might be worth the hoops you have to jump through to get the animal companion; if not, it isn’t.
Dragon Rage (Eberron Campaign Setting)
Requires Dragon Totem, so two feats, and with both you get resistance 15 to a type of energy damage. Acid and electricity are both options, so this can complement Blazing Berserker and Frozen Berserker pretty well, but 15 is a pretty small number.
Dragonmark Rage (Dragonmarked)
With thanks to @Miniman for pointing it out.
Get fast healing while raging, which is definitely on-brand, but the numbers are much too small. Still, it does have some potential for the “rage-cycling” build, see below.
Chaos Rage (Dragon vol. 326)
Practiced Spellcaster but for barbarians. Most of the time, it’s worth +1 rage/day—and thus half what Extra Rage does, and you should take Extra Rage instead. If you happen to be within 4 levels of greater rage, however, it’s a feat to get greater rage—which is well worth considering. The problem is, well, you shouldn’t be within 4 levels of greater rage most of the time, because barbarian class levels are mediocre after 2nd and few PrCs stack directly like that. Maybe a champion of Gwynharwyf would take it or something.
Much better for berserker strength, though, since it’s up to +20 to your threshold. See “rage-cycling” below for details.
Channeled Rage (Races of Destiny)
Add your Strength bonus to a Will save by spending a daily use of rage. Uses of rage are somewhat painful to come by, and the feat requires being a half-orc which is miserable. Plus we kind of want to ditch our Strength bonus if we can...
Cobalt Rage (Magic of Incarnum)
Insight bonus to Will saves while raging equal to essentia invested in the feat. It’s not really all that great (you can get decent-sized bonuses but you have to invest pretty heavily to get there; on its own the feat offers a mere +1 which is utterly trounced even by Iron Will), but it gets a lot better if you were multiclassing totemist anyway, which is a solid choice.
Reckless Rage (Races of Stone), Ice Troll Berserker (Unapproachable East), Stone Rage (Races of Stone)
Are bad, don’t take them.
Righteous Wrath (Book of Exalted Deeds)
While raging, you maintain clarity of mind unusual among barbarians. You are perfectly able to deal nonlethal damage, stop your attacks to show mercy, and distinguish friend from foe even in the heat of your rage.
These things? These are things a barbarian has absolutely no trouble doing in the first place. You don’t need this feat—it doesn’t let you do anything you couldn’t do otherwise.
That said, if your DM gets picky about “patience or concentration,” and/or doesn’t get picky about what Righteous Wrath says it actually does, being able to “maintain clarity of mind unusual among barbarians” may be valuable. Might be worth asking about, or keeping in mind if there’s anything that you want to do that they say won’t fly in rage—you can always pull out “well, what if I take Righteous Wrath?”
The other exception might be for a frenzied berserker. While in a frenzy, you do have difficulty doing the things that Righteous Wrath helps with—but it’s not clear that Righteous Wrath applies, because it says “rage” and frenzy says, well, “frenzy.” But if your DM allows Righteous Wrath to eliminate frenzied berserker’s problems, then you should obviously take it—honestly, it would be rude not to.
“Rage-cycling” refers to restarting a rage in order to refresh benefits and features that only happen once per rage. Rage and most of its variants prevent it by blocking a barbarian from starting a second rage in the same encounter as a previous one, but as topquark’s excellent answer points out, berserker strength doesn’t have that line, and it doesn’t have any daily limits on how much it can be restarted.
The obvious problem is that berserker strength also doesn’t let you directly choose when to start and stop it, but topquark has a solution to that, too. Berserker strength ends only if healed above the threshold. If, instead, you get more hp by increasing your Constitution, it doesn’t end. So in such a situation, we can maintain the berserker strength as long as we aren’t healed, and then end it by getting any healing at all, since we’re already above the threshold.
How to increase Constitution? Ettercap Berserker’s got that handled, giving a juicy +6 Con when berserker strength activates. Not only does this put you in the “any healing ends berserker strength” territory, if you line things up just right, losing that Con can put you below the threshold again—restarting berserker strength immediately and automatically when you get any amount of healing.
There are two potential difficulties here:
Our hp is too low, and the healing isn’t sufficient to end berserker strength and trigger a cycle.
Our hp is too high, so that even when we lose the Constitution from Ettercap Berserker, we don’t dip below the threshold and trigger it again.
Of the two, the latter is worse: it means we are left out of our rage, without any of the defenses built into our rage. But it’s also the one we can do something about. For instance, consider a 5th-level druidic avenger with Constitution 8: they have, on average, 21 hp. Their berserker strength hp threshold is 25. They literally cannot be above the threshold in their base state, and are always in berserker strength. Ettercap Berserker then keeps their hp up, and any healing triggers a rage-cycle.
And we can avoid having to have such poor starting Con by taking Chaos Rage—where normally that feat is OK, at best, here it effectively means multiclassing is actually a legitimate option. That’s good, because we desperately need bonus feats and druidic avenger doesn’t get any.
With fast healing, that’s a rage-cycle every round. Unfortunately, that can be tough to get, and to do it well is going to take 2-3 feats, or a ridiculous amount of gold.
Troll-Blooded (Dragon vol. 319, with thanks to @Forrestfire for pointing it out), which actually gets regeneration 1, which combines really nicely with Blazing Berserker and Dragon Rage, but Troll-Blooded requires Toughness.
Dragonmark Rage also does it, and that’s always on thanks to the way we have berserker strength set up, but you have to take Least Dragonmark or Aberrant Dragonmark to qualify.
Lesser vigor is a 1st-level druid spell that applies fast healing 1 for over a minute, and works really well in a wand, but to zap a wand we need Rage Casting and its requirements, Combat Casting and Quicken Spell, so 3 feats instead of 2 for Troll-Blooded or Dragonmark Rage. The advantage is that you can also use other wands, and Wand Surge exists; see below.
Healing blood (Lords of Madness) is the most overpriced item in the game. It’s a graft, so it’s permanent and non-magical, but it’s also just fast healing 2. And it costs 182,000 gp. But no feats. Feats are valuable, but they aren’t worth 90k gp.
However we go, the rage-cycling makes Mad Foam Rager an incredible defense, likely better than even Blazing+Frozen Berserker. More importantly, Raging Luck means we get an action point every round. How can we use those?
Wand Surge (Magic of Eberron): spend an action point to activate a spell trigger item instead of spending a charge, and Rage Casting allows us to do it during a rage. Free zaps of a wand, scepter, or staff, once per turn. You can definitely go far with this, plus it guarantees that you can zap lesser vigor every minute of every day and never run out of charges.
Takes 4 feats on top of Ettercap Berserker, so 5 total, but solves fast healing for us while simultaneously providing us many options for combat.
Chaos Rage becomes even more important, because dipping cleric gives access to the Magic domain and effectively eliminates UMD as a concern for us (cleric, druid, sorcerer, and wizard spells are all usable without UMD for us). Consider Madness for the other, so you can get Frantic Rage at some point.
It moves away from the “no-attacking” thing, but Eilservs School (Drow of the Underdark) definitely warrants consideration as a very weird—but potent—gish build.
Heroic Channeling (Faiths of Eberron): spend an action point in place of a turn/rebuke undead attempt to activate a divine feat. Requires getting divine feats, of course, but there are quite a few good ones, and several that look very nice to activate for free every turn. Note the existence of the bone talisman spell allowing a druid to turn undead, allowing them to qualify for this feat and any divine feats.
Takes 2 feats on top of Ettercap Berserker (itself and some divine feat to power), but also still needs to solve fast healing which is going to take another 2 feats, so it’s the same 5 feats that Wand Surge needs.
Protective Mark (Dragonmarked): DR 10/magic-and-your-choice-of-bludgeoning-piercing-slashing for 1 round, every round.
Jungle Veteran (Secrets of Xen’drik): Act in a surprise round you’d otherwise miss. Might actually counter the dire tortoise, amazingly (though it cannot perform that trick itself).
Dragonmark Spellturning (Dragonmarked): Turn a spell, as per spell turning, as an immediate action. Limited by the quality of your dragonmark, though. Still, between this and Mad Foam Rager, that’s a lot of stuff you’re shaking off.
Heroic Channeling and/or Wand Surge seems a must here to me, and either one is going to require 5 feats total. With flaws, as a human, possible by 3rd level; reasonable. Without flaws, as another race, not possible until 12th level; unreasonable. Multiclassing hurts badly because it lowers our berserker strength threshold. And remember, we want a bunch of other feats, like Blazing Berserker, Frozen Berserker, and Mad Foam Rager. More divine feats to go with Heroic Channeling, any of the other three action point feats I mention, and so on.
In short, this build is probably best for E6, played with flaws or after you’ve gotten at least a couple of bonus feats after 6th. There’s so much it can do with feats.