Fable 2, Far Cry 2, COD4, GTA IV (That's prefferential), Halo 3 (Gets boring quickly though
), Assasin's Creed (Pretty repetitive), BioShock, Guitar Hero: World Tour, Gears of War 1 and 2 (when it comes out), Tom Clancy's Endwar....that's all that comes to mind at the moment.
Overall....you'll want a............................Xbox Live subscription
It gets pricey though so i'd suggest spreading the purchasing out.
You can also as Warlord stated get them off Ebay for extremely cheap prices.