A LOT of stuff although that kind of conversion is surprisingly tough. A lot of the games I really love are especially hard to figure out a good way of making into an anime like Doom's ultra violence and Dark Souls' deliberately ambiguous method of storytelling and world building are tricky to nail in conjunction with character growth and story arcs without sacrificing the identity of the original works (The Doom film being an excellent example of this as it fails in almost every regard, which sucks considering its inspired by Doom 3 rather than 1 and 2 - you can do a LOT with Doom 3's sci-fi horror set up). The only two "game to anime" conversions I know of from the top of my head that somewhat succeed are Devil May Cry and the sadly overlooked SiN, though they're both FAR from perfect.
A Digital Devil Saga anime adaptation would be nice. Condemned or Condemned 2: Bloodshot would make an interesting horror. Dark Cloud, Skies of Arcadia and Baten Kaitos would be so much fun, I imagine!
I would love to see Portal as an anime