Team Fortress 2!! One of the best games of all times. In case you don't know what it is, it is a multiplayer shooter with two teams: Reliable Excavation and Demolition (RED)
and Builders League United (BLU). They are constantly battling except in Mann versus Machine (Mann co. supposedly supplies all the weapons and hats), which in that the teams must join forces to fight the waves of attacking robots. There are nine playable classes : Scout, Soldier, Pyro, Demoman, Engineer, Medic, Sniper, and Spy. It is not as military based as it sounds. To learn more go to the wikipedia page or download the game on steam (It's Free!).