My DM has some evil dice. These dice seem to roll a much higher than average number of 20s, resulting in a much higher than average critical hit rate against the party. What are some ways a character can protect themselves vs critical hits? My party's current average level is 6, but solutions at any level from 1-20 are welcome. I'm already aware of the fortification magic item properties.
Answers, in order from mechanically-cheapest to mechanically-priciest:
The easiest way is to be a Necropolitan from Libris Mortis: you lose a level as if you’d been killed and had raise dead cast on you, and then lose 1,000 XP on top of that. There’s also some gold involved. In exchange, you become Undead, but lose nothing but your Constitution score.
You are now immune to an enormous smorgasbord of things, and if you dumped Constitution to begin with, you now have effectively a better Point Buy than others. Be careful about your Fortitude save and about Turn/Rebuke Undead – those are weak points for you.
The easiest way to get the Construct type is by being a Warforged (Eberron Campaign Setting), but they have the Living Construct subtype which removes immunity to critical hits. They can fix that, however, by taking the Warforged Juggernaut prestige class from the same book, or by having construct essence from Races of Eberron cast on them (construct essence is a 5th-level Artificer infusion and a 5th-level Sor/Wiz spell; there is also a greater construct essence as a Sor/Wiz 9 spell).
Some other types (e.g. Elemental, Ooze, Plant) and subtypes (e.g. Swarm) also provide immunity to critical hits. These are not nearly as easy to come by as Construct or Undead however; I mention them more for completeness’s sake than as a serious recommendation.
These are effects that give you a % chance to ignore a critical hit (or Sneak Attack). Light fortification is a +1-equivalent for a 25% chance to ignore them, moderate fortification gets 75% for a +3-equivalent, and heavy fortification gets 100% (i.e. immunity) for a +5-equivalent. Ideally, you get a +1 heavy fortification buckler or something, for the minimum 36,000 gp.
Note that all Warforged, not just Juggernauts, get Light Fortification.
Be ready to read a lot of options
Critical hit immunity/protection against critical hits comes in a LOT of flavors in 3.5, which you seem to be playing (since that's the tag on the thread). The easiest method, by far, is to just ditch life and become undead. The Necropolitian template (Libris Mortis) only costs you a level - not a +1 level adjustment, a level, which is easy to gain back - and in exchange gives you the Undead type with its entire suite of immunities and bonuses. Necropolitian doesn't alter your alignment, but talk with your DM about it anyway - as you should for all character options.
Another option is to try and protect yourself from attacks in general. Non-AC defenses such as Mirror Image, Displacement, Blur, Blink, teleportation, flight, and burrowing are great for that, since you can't be struck by a critical hit if you aren't struck in the first place. The Wall of Blades maneuver lets you intercept an attack, and there are some spells and/or class features that can force re-rolls on an enemy.
You might be tempted to go for something like Ysgardian Heartwire (Book of Exalted Deeds) but generally speaking trying to boost your AC is a trap that you shouldn't fall into. Fortification is a pretty good option as far as armor enchantment goes, especially if you can tag it onto an Animated shield (this assumes shield proficiency). Debuffing enemies can help lower their chances of hitting or confirming their criticals - spells like Slow, Blindness/Deafness, Glitterdust, and their ilk prevent an active enemy from hitting/critting effectively.
Some prestige classes grant crit resistance or crit immunity, most notably the Warshaper prestige class from Complete Warrior. If your concept or build permits, Warshaper is a great PrC for melee shapeshifters. A bit of splat-diving can find most of those for you, but the options there are too numerous for me to actually list.
If you're a Druid or a Wizard/Sorcerer, the Heart of Air/Earth/Fire/Water spells from Complete Mage will do it. If you have two of them active at once (doable at level 5) you get Light Fortification. If all four are active, you gain crit immunity (not doable until level 9 as Heart of Fire is a 5th level spell).
The nice thing about these spells is that they all have some other effect while they're up, so they're worth casting normally anyway. Heart of Earth for example grants 2*Caster Level temporary hit points. You can also use a swift action to get rid of it, and give yourself a Stoneskin spell temporarily instead (very handy if you suddenly find yourself swarmed!).
Okay, there's something KRyan and Lord_Gareth haven't mentioned yet.
Be a plant. Plants are immune to critical hits, period. Volodni (Unapproachable East), Needlefolk (MM2). Some specific spell also did the job, but I can't remember it, though Polymorph, Polymorph Any Object and Shapechange are fine, too.
Polymorph spells, Iron Body (PHB, 8th level) and Stone Body (PGtF, 6th level) also do the job.
Polymorph Any Object will also do the job.
Well, aside from polymorph spells I can't really remember anything, though there was some specific spell which granted ooze type.
Master of Many Forms gives you Ex special qualities of your new form starting with 7th level and that includes immunities. He can take Plant, Ooze and Elemental forms on 7th, 8th and 9th levels, respectively.
To be continued
The lowest-level way is likely the 2nd-level cleric spell living undeath [necro] (Spell Compendium 134) that for 1 min./level renders a touched creature immune to critical hits and sneak attacks; for the spell's duration, the creature also suffers a −4 penalty to Charisma (minimum Charisma 1). An adventurer can purchase in a small town a potion of living undeath (2nd-level spell at caster level 3) (300 gp; 0.1 lbs.) or purchase in a small city a wand of living undeath (2nd-level spell at caster level 3) (4,500 gp; 0 lbs.). The spell's probably not a great choice for sorcerers, most bards and paladins, and some clerics, but it's better than dying to the DM's lucky dice.
Higher-level alternatives to the spell living undeath include the 3rd-level Sor/Wiz spell permeable form [trans] (Lords of Madness 129) yet its short duration makes it impractical, the 4th-level level Sor/Wiz spells polymorph [trans] (Player's Handbook 269) when used to assume a the form of a type of creature that's immune to critical hits like an ooze or plant and sandform [trans] (Sandstorm 119), the 5th-level Drd spell plant body [trans] (SpC 159), the 6th-level Sor/Wiz spell stone body [trans] (SpC 207), the 7th-level Sor/Wiz spell elemental body [trans] (SpC 78-9), the 8th-level Sor/Wiz spell veil of undeath [trans] (SpC 229), and the 9th-level Sor/Wiz spell shapechange [trans] (PH 277-8), the spell polymorph's much bigger brother. There are more besides these; I just wanted at least one example per spell level above 2nd.
While it's probably beyond the means characters below level 8 (who'll often have more immediately useful things on which to spend their wealth), the talisman of undying fortitude (Magic Item Compendium 188) (8,000 gp; 0 lbs.) allows the holder 2/day to take a swift action to become for 3 rounds, in addition to other effects, immune to critical hits. (This DM has found the talisman also vital for keeping alive foes of his high-level PCs.)
For the sake of posterity, here is a list of items which anyone can buy (assuming they are for sale) that will provide you with immunity to critical hit or a percentage chance thereof.
Fortification Armor or Shield (DMG): Pretty much the textbook answer. If your shield is only a +1, and then enchanted with the best Fortification you can afford, the cost will be 4k for 25% fortification, 16k for 75% fortification, or 36k for 100% fortification. Even if you're a squishy spellcaster, a mithril buckler has no spell failure chance, no check penalty (for lack of proficiency) and weighs almost nothing. This is the best option for most characters.
Gemstone of Fortification (Draconomicon): This is an option which is slightly trickier to manage (Unless you are a true dragon, or can claim similar scales and gemstone embedding traditions, it requires a Limited Wish to embed it into your skin). It does, however, have the benefit of not taking up a slot, which can be quite nice if you need other armor properties (such as Greater Blurring for a rogue, or Beastskin for a Druid) on your armor, or if you are somehow unable to wear armor at all (ie, you are a monk). It costs 3k for 25% Fortification, 15k for 75% Fortification, or 35k for 100% Fortification (plus the cost of the Limited Wish).
Ring of Armor (City of Splendours: Waterdeep): This option takes up a slot (Ring) which has a fair bit of competition. Also, it is a great deal more expensive. Even if you have +5 in other armor properties (+1 Soulfire, maybe?) the armor property is cheaper. Nevertheless, for the record it costs 8k for 25% Fortification, 32k for 50% Fortification, and 72000 for Heavy Fortification. Each ring also gives you a +1 Deflection bonus to AC.
Given the rules in the MIC for combining the 'big six' with magic items, I would talk to your DM about using the pricing for the Gemstone of Fortification+the cost of a ring of protection. It will still eat a slot, but that is a much more reasonable cost than this white elephant.
For temporary protection from Critical Hits (and more affordable) the following item exists:
Talisman of Undying Fortitude (MIC): Slotless Item (Held) which for 3 rounds, twice a day gives you immunity to crits as a swift action, along with a whole host of other undead style immunities. It's only 8k.
All of these options exist for anyone with the cash. No need to be a spellcaster.
I will list one additional item, for Sorcerers in particular. A Runestaff (MIC) which contains Heart of Air, Heart of Earth, Heart of Fire, and Heart of Water (Complete Mage), each usable 3 times per day costs 15,800 gp. This is both a thematically nifty and /really useful/ runestaff to have, as a sorcerer. And it gives you Caster Level*3 hours of immunity to crits (along with a bunch of other cool effects) for less than 16k and some spell slots.