Quote
Alien Swarm is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community. Available free of charge, the game thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics. With your friends, form a squad of four distinct IAF Marine classes.
http://store.steampowered.com/app/630/
I haven't played it yet myself thanks to a slow download speed, but it looks pretty neat and best of all its completely free. Who wants to play?