There are a couple games I play coop in addition to the stuff that has already been discussed here.
Lego Star Wars: Complete Saga - Obviously this is a more family oriented game, but I really enjoy it. All of the Lego games are very fun. You can play 2-player coop, either on the same console, or over Live. The odd thing is, Lego Star Wars was the first game that Travelers Tales released in their Lego series, and it had coop. Every one since then has not. Real bummer, because Lego Indy and Lego Batman are better games. But, Lego Star Wars is good fun.
Crackdown - This game is one of my favorite 360 games ever. It is just a load of fun to run around the game world blowing crap up and just generally destroying everything. The game is even better when you get 2 players in it. You have access to the entire game world at all times, so you can either team up on objectives or split up and accomplish them quicker. A very highly overlooked game, and one that is great fun in coop.
Crackdown 2 is coming out in a few weeks, and it looks to just expand on everything from the first one. The story mode can now do 4 player coop, and they are adding a 16 player competitive multiplayer. I can't wait.