assassin's creed, bioshock and mass effect are very fun games. mass effect feels like KOTOR with guns. bioshock is possibly one of the most engrossing games of this generation thusfar (but it's cheaper on Steam irregardless of the system - they had a sale where it was 5$ ).
i'd add mirror's edge to bardic's list - it's basically a platformer, but it's first person and has some phenomenal graphics. really fun to play well.
i noticed you didn't have some of the FPS games that have helped the 360 sell so much in there - halo 3, gears 1 and 2 (2's better, but 1's got a lot there), and RE5 - but there's some other genres that are excellent. star wars force unleashed is currently 15$ at gamecrazy, and is probably the best action star wars title to date (yes, i think it's better than battlefront). i put a lot of time into chronicles of riddick (FPS, but there's easily 30 hours of gameplay there), and halo wars was fun while it lasted although it's short. i enjoyed wheelman, which is a racing/action game with a few letdowns. gta4 was one of the most immersive sandbox games to come out recently, and the DLC for it (lost and the damned) is almost 20 hours of gameplay for 20$. overlord was incredibly entertaining, and crackdown and saints row 2 were both sandbox games that featured over-the-top entertaining violence and general badassery. don't forget fable 2, which was incredibly entertaining as well, and had some really fun DLC. fallout 3 is incredible if you don't have a pc, and the DLC for it is incredibly entertaining and really extends the game well beyond the initial disc.
if you feel like spending, prototype was off the chain. it's SO much fun, a sandbox game with superhero powers but no morality system. excellent.