Alien Swarm

Alien Swarm is a game and source SDK developed by a group of talented designers at Valve who were recruited by the mod community. The game is available for free and plunges players into an epic bug hunt with a unique blend of co-op play and squad-level tactics. Form a squad with your friends from four different IAF naval classes. Plan your attack against a variety of aliens with an unlockable arsenal of weapons and countless equipment configurations. Make your way through an overrun off-world colony and destroy the alien infestation in environments ranging from the icy planet surface to an underground lava-flooded mining facility. Along with the game, you'll receive the complete codebase for Alien Swarm, which includes updates to the source engine and SDK. Alien Swarm adds a 3rd-person camera, depth of field, improved dynamic shadows, and a variety of gameplay enhancements to the Source engine.