Don't forget the incredible Full Spectrum Warrior! It can be had for as little as $20 now. One of my favorite co-op XBL titles. Too bad the DLC mission is only single player.
Then there is the usual suspects: Ghost Recon, Ghost Recon:Island Thunder, Rainbow Six 3, and Rainbow Six 3: Black Arrow.
I wish more developers would start making more co-op online games. I perosnallyl can't stand split screen, especially on a console with a built in LAN port. I just think personallyl that too many developers ar just being lazy, especialyl in regards to multi-platform titles. But I digress......
Check out Full Specturm Warrior and drop me a line. I'm always good for a game!