Don't just limit the scope of these ideas to CRPGs... there's tons of cool creative content out there in really good action/strat/adventure games, ready made for theft!
Thief (The Dark Project & Metal Age): The sheer style factor of this game series makes a compelling argument for a medieval fantasy-punk-ish setting (its not really steampunk), IMO. I'm currently running a Greyhawk campaign, but I have homebrew I've been working on for the future, and the Thief game setting and story elements are having a significant impact on my homebrew.
For example, in one of my older cities, there is now a walled-off district called the 'Old City' which was overrun with a strange 'plague of undeath'. This thoroughly haunted district is strictly quarantined with a special temple-trained guard force to make sure nothing (corporeal or otherwise) gets in or out. However, generations later, there remain long persistant rumours and tales about the riches left behind in hastily abandoned temples and mansions of the Old City. Yummay.
Deus Ex: In a similar fashion to the Thief games, Deus Ex provides a compelling setting and characters that could easily be used in a dark future d20 Modern/Future/M&M style game. In fact, I think it could be interesting to bring story elements of Deus Ex into a fantasy setting. Morgan Everett and the Illuminati become an ancient, secret guild arcanists fallen onto hard times (think: amoral Harpers). Bob Page and MJ12 become a splinter group dabbling in necromancy and alienistic magic (perhaps Walton Simons is an advanced, experimental vampire capable of tolerating daylight - pseudonatural or abberant templates could be useful, too). The NSF become an organized crime guild with revolutionary tendencies, supported by the renegade wizard Tracer Tong. UNATCO becomes a royal inquisitional paramilitary guard (
à la musketeers as law & order zealots). Etc. Great fun.
Half-Life: Following the
Incursion theme, you've at least got some fun abberation-type monsters here. Use a Cthulian spin to make it more fanstasy, less X-Files - or stay in-genre and use d20 Modern/Spycraft/etc. HL2 looks like it will advance a richer story line than the spare-but-fun threads of the first game. When 3E first came out, I considered writing up the Half-Life critters in d20 monster stats, just as a fun exercise in monster building. I may still try that sometime.
Diablo (and Diablo II): Not only are there lots of little bits of classic fantasy gaming fun in here, there is actually a pretty cool setting/backstory. It's easy to ignore in the game, but if you look for it, there's a moderately fleshed-out setting outline available here. It's more obvious in the second game. Story elements like the Sin War and the Haradrim are ready-made for D&D. I was really dissapointed that the D2 D&D supplement was basically the computer game as a PnP campaign, rather than a setting book. I'd still like to see that.
Myth: This real-time wargame series has a superb atmosphere and story. Lots of great monsters and BBEG type villains. Reminds me
alot of the Black Company novels. Check out
http://myth.bungie.org/legends/index.html for a ton of setting info.
Interstate '76: An alternate history setting in the American Southwest could be a cool retro Car Wars/d20 Modern/Wild West meets Muscle Cars game. Would definitely needs some good vehicle d20 rules (Spycraft?).
Dark Forces/
Jedi Knight/
KotOR: Duh!
EDIT: doh.., almost forgot...
Gabriel Knight: As an example of a great adventure game series, there's a ton of cool plot and story material available here. Just the
Schattenjäger concept alone is worth the price of admission.