Check out Evolve, which everyone thought was going to be as big as L4D but just didn't quite click with the masses. It's available for free now on Steam I think. Army of two was litterally built around the idea of co-operation. Um....Dead by Daylight is a asymetrical horror game where you can play either the killer chasing down people or one of the group of hapless victims who need to work together to escape the killer. Imagine if you could play as a one of those slasher movie villains or be stuck fleeing from them helplessly trying to figure out how to get out of there and you get the idea of the game.