Playing games cooperatively is great fun and can make even the most worthless game seem playable so here is the list of games I enjoy to play co-op in. This list will be updated as I find new games and for a full list of games that support co-op visit Co-optimus. :).
Borderlands Game link
First-person shooter role playing game with four player co-op. You can play with your local game character in other peoples game.
Call of Duty: Modern Warfare 2 Game link
Special ops mode support co-op with another player.
Saints Row 2 Game link
Grand theft auto clone that is better then the source material and it also has two player co-op play of the whole campaign with drop in play.
Swat 4 Game link
Tactical shooter that really demands teamwork of all those playing. Unlike the Rainbow six series not everyone on every map has to die as you are working for the law and it’s always best to take everyone alive.
Left 4 Dead / Left 4 Dead 2 Game link
You and three other players (or computer characters if you do not have enough players) try to survive a zombie onslaught.