Everybody agrees on one spell
I read about 20 threads on the feat Craft Contingent Spell (Complete Arcane 77). To summarize, these threads usually have one side that says, "The feat's so broken it's never been allowed in a campaign I've been in, but here's how I'd break a campaign with it were it allowed," and another side that says, "It's a very good feat, but it doesn't break games if it's managed responsibly." Despite this chasm, both sides agree that if the Craft Contingent Spell feat is available then everyone should get a contingent spell of revivify [conj] (Spell Compendium 176) (5th-level spell at caster level 9) with the trigger If I die (5,500 gp; 0 lbs.). Other necessary contingent spells are up in the air for two reasons.
First, the Craft Contingent Spell feat is so impactful that for verisimilitude either the feat's effects must be known, obvious, and game-changing from the campaign's outset or taking the feat and buying contingent spells is possible only for a rare few.
Second, the Craft Contingent Spell feat requires DM adjudication. The description of contingent spells, in part, says, "Triggers for contingent spells are usually events that happen to the bearer of the spell," then provides an unnecessarily long list of those events without offering an unusual example (CAr 139 and emphasis mine and see below). That pregnant usually forces DM involvement.
For example, the community often recommends a contingent spell of celerity [trans] (Player's Handbook II 105) (4th-level spell at caster level 7) with the trigger If an enemy would take an action in combat before I would (2,800 gp; 0 lbs.). In other words, when the bearer of the contingent spell of celerity loses initiative to a foe, the bearer still goes first, typically casting a spell that ends the encounter.
However, canny inhabitants of a setting where even a reasonable Craft Contingent Spell feat exists (i.e. usually doesn't mean always) should either themselves have that same contingent spell of celerity (or better or more or all three) or have developed practices, procedures, tactics, and techniques to deal with contingent spells generally—and probably both. Hence the DM must mentally game scenarios involving a variety of contingent spells, both alone and in combination. Further, if contingent spells aren't secret, the DM should inform players beforehand of tactics that are commonly used to optimize and thwart contingent spells. In addition, that contingent spell of celerity has an unusual trigger. Losing initiative to a foe isn't one of the description's trigger examples, and I imagine most folks don't view losing initiative to a foe as an event that happens to the bearer, therefore the DM must find that trigger acceptable, and there's no guarantee that a particular DM will.
This combination of setting and DM intervention makes general recommendations for contingent spells extremely difficult. Certainly, a list could be assembled of what contingent spells a character should have in the most restrictive environment. For instance, let's say that the DM rules that usually means always, and the DM rules that the description's examples are inclusive (which is dumb but maybe he brings snacks). Then a contingent spell's trigger is limited to the contingent spell's bearer dying; contracting a disease; being exposed to a breath weapon or to energy damage; falling; being exposed to poison; being exposed to a dangerous environment (in a fire, underwater, and so on); being affected by a sleep or fear effect; gaining one or more negative levels; and being rendered helpless, deaf, or blind.
To be clear, that's still not a bad list, and even with the feat at its most restrictive at least one PC in the party should still take it. Yet to stay within the DM's strictures means that the choices for contingent spells are obvious. You get a contingent spell of remove disease [conj] (PH 271) (3rd-level spell at caster level 5) with the trigger If I contract a disease (1,500 gp; 0 lbs.), a contingent spell of resist energy (fire) [abjur] (PH 272) (2nd-level spell at caster level 3) with the trigger If I would take fire damage (600 gp; 0 lbs.), and so on. You don't need this answer for a list like that; a decent list can be made by just flipping through the Player's Handbook.
What this answer wants to help you with and can't is when your DM says, "The Craft Contingent Spell feat is available in this campaign, everyone uses it, and any trigger appropriate for the 6th-level Sor/Wiz spell contingency [evoc] (PH 213) is also appropriate for a contingent spell." For answers to suggest spells for that DM's campaign, you'd need to provide what else the DM has said about the long-term impact of the Craft Contingent Spell feat on the setting—what tactics are used, what tactics are obsolete, and so on—, as well as what triggers the DM has determined are appropriate for the contingency spell (q.v. here). In short, there's pretty much no way to assemble a list of optimized contingent spells in a free-for-all environment without context. If the DM does provide context, I recommend posing another question specific to that DM's campaign with that context. Then answers can dig through sources and make recommendations for that campaign.
"So you want a campaign?"
In a comment, user Dan B called me on my judgment, saying that leaving all this to an individual DM was a copout on my part, adding, "If you made some reasonable assumptions about the setting and sketched out what sort of spells would be good as a result, this would be a more interesting and useful answer." I agree that it would make this answer more interesting, but I don't think it would be more useful. What's reasonable at one table totally isn't at another. Nonetheless, let me provide an example of one way that a DM could have a campaign wherein contingent spells are present and desirable but don't dominate the campaign.
PLAYER: Now that my wizard's level 12, I'm thinking I'll take the Craft Contingent Spell feat.
DM: Are you sure?
DM: Have Ygar make a Knowledge (arcana) check.
P: Okay. [Rolls.] 10?
DM: Did you only put one rank in that?
P: Shut up.
DM: It's enough. Professors made a big deal out of it in magic history class. Anyway, there was a time when every caster beyond a certain point took the Craft Contingent Spell feat, and they all used it for the same thing: contingent spells of celerity. Their first contingent spell of celerity would have the trigger If an enemy would take an action in combat before I would and each successive one would have If an enemy's first (then second then third and so on) contingent spell of celerity would trigger until their Hit Dice could accommodate no more. So, like, a level 17 caster would have nearly 50,000 gp invested in 17 contingent spells of celerity.
P: That's so dumb. And shockingly plausible.
DM: Anyway, whenever a caster would fight another caster, each'd trigger contingent spells of celerity until the higher-level caster triggered one more than the lower-level caster. This high cost led to casters pumping their initiatives and fighting noncasters. Still, lone casters (and their contingent spells) were especially vulnerable to attacks by groups of casters and to hit-and-run attacks, so wizards formed guilds for protection.
P: From each other.
DM: And of their investments. And to have a safe place to reup their own contingent spells. It takes 3 days to create a contingent spell of celerity, and not a lot of ways to cut that down are allowed in this campaign. Anyway, magical advancement stagnated because casters only cared about initiative and contingent spells. Anything else was pointless.
P: The top of the adventurer food chain worried almost exclusively about going first? So it was boring and competitive.
DM: It was a different time. Lots of paranoia, backstabbing, and unconventional tactics. The dinosaur stampede that destroyed Xjabse in '82 was started by a group of mundanes to divest the town's casters of their initial contingent spells of celerity, for instance.
P: Ygar got a 10.
DM: Sorry. Forget I mentioned the dinosaurs. Anyway. Eventually the casters agreed that this whole nested-contingent-spells-of-celerity setup was a massive waste of resources. Plus, it was a magical dead end—or, more properly, a treadmill that if you didn't get on then you lost, and if you got on then you ran until you lost anyway. So a group of casters got together and broke the treadmill.
DM: Ygar doesn't remember. Something about multiple wishes and miracles and bargains struck by high-level casters with the council of inevitibles and the gods of magic. The result was that contingent spells in the campaign have some house rules:
- A creature can't have the same spell more than once as a contingent spell.
- A creature can't have the same or similar trigger on more than one contingent spell. I'll just tell you when considering triggers if they're similar. No surprises.
- A creature's total spell levels of contingent spells with unusual triggers can't exceed the creature's summed unmodified caster levels gained exclusively from class levels. For example, a typical human Wiz12 can have a 6th-level contingent spell, and a 5th-level contingent spell, and a 1st-level contingent spell each with unusual triggers, but that Wiz12's remaining 9 contingent spells must have a usual trigger.
- If one or more creatures sharing a space have contingent spells with the same or similar triggers (again, I'll tell you), and one contingent spell is triggered, all contingent spells with that trigger present among creatures sharing that space are triggered also but have no effect. If they'd all trigger simultaneously, the DM rolls randomly to determine which one actually triggers.
- A contingent spell has a minimum base price of 1,000 gp. A contingent spells that would have a lower base price can still be created, but its base price becomes 1,000 gp.
This last is why so far in the campaign you've only seen a couple of folks with contingent spells instead of every peasant with a contingent spell of cure light wounds [conj] (PH 215–16) (1st-level spell at caster level 1) with the trigger If I'm dying (100 gp; 0 lbs.).
P: That's… a lot of rules.
DM: I know. O, and one more thing. I mentioned this when someone asked a couple of levels ago, but if you want to straight-up buy a contingent spell, you've to role-play doing so. "Anything having a price under [a town's gp] limit is most likely available, whether it be mundane or magical" (DMG 137), but contingent spells are an exception in this campaign.
P: Is that it?
DM: Until Ygar puts a rank into Knowledge (arcana), makes another check, and gets higher than a 10. Then I can tell you more about the dinosaur stampede.
First, the DM should have provided some of this information as part of the campaign's initial guidelines, but if the DM is being generous in allowing Dungeons & Dragons 3.5 material into the campaign, the sheer volume of it all means that even world-altering game elements may slip through the cracks, so some slack should be cut.
Second, some readers are thinking, "I'd never take the Craft Contingent Spell feat under those house rules," and other readers are thinking, "Those are reasonable house rules; I can work with those," and still other readers are thinking, "Foolish DM, I will turn your campaign into a smoking ruin." However, keep in mind that every game needs some kind of gentleperson's agreement so that everyone—including the DM—has fun.
So these house rules would be instead of banning the feat. If these house rules would cause you, dear reader, as an imaginary player, to reject the Craft Contingent Spell feat out of hand or try to subvert the campaign, then that DM wouldn't've made those house rules (or, at least, be open to negotiating them). That DM knows his players; I don't know you.
And, despite all this, I still can't provide a list of common or optimized contingent spells for the above campaign because I'm not running that campaign. Were I, then I'd go through all the campaign's spells and determine which contingent spells and their triggers would be reasonable, affordable, and interesting and provide that list to my players, but that list would be useless in any other campaign because another campaign will have dealt with contingent spells completely differently. I'm sorry, Dan B and J. Mini, that I can't be of more help.