I'm a sucker for cheap games (Under $20) so here are a couple that you may like:
-Marvel Ult. Alliance (Very fun hack and slash)
-The Orange Box (Very cheap now and has sooo many good games. If you don't already have the games in it, then you should for sure get it)
-Dead space (A little scary, and not under $20, but a very good game)
-Viva Pinata (Childish, yet oddly extremely addictive)
-Army of Two (GREAT game to play with a buddy, HORRIBLE game alone)
-Phantasy Star Universe (If your into MMO's this is a great console one. Monthy fee though =/)
-Chromehounds (Very fun if you have enough people to play with)
-Kameo (Only about $10. Ok game)
If you get Live here are a few arcade games to check out too:
-Peggle (One of the best XBLA games to date)
-Castle Crashers (Very fun, especially with a couple friends)
-Blazing Birds (Had a blast with it, I would try the trail first)
-Aegis Wing (Free and fun)
-The Maw (No replay value, but great game)
-Geometry Wars Evolved 2
And that's about it. Somewhat long list, but all of those games except Dead Space are around $20 so you should for sure check them out.