Valve's Free Shooter

"Alien Swarm is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community." So says Valve. Oh, it also says that the games is "Available free of charge..."

That's correct, it's free... as is the "complete code base for Alien Swarm that features updates to the Source engine as well as the SDK. Alien Swarm adds 3rd person camera, depth of field, improved dynamic shadows and a wide variety of gameplay additions to the Source engine."

"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. Plan your attack using an unlockable arsenal of weapons with countless loadout configurations against a wide variety of aliens. Blaze your way through an overrun, off-world colony, eradicating the alien infestation in environments ranging from the icy planet's surface, to a subterranean lava-flooded mining facility."

This is what you get:

Tactical, 4 player co-op action game with a top-down perspective
Complete game code and mod tools
Unlock persistent items by gaining levels
Over 40 weapons and equipment with countless loadout configurations
4 unique classes and 8 unique characters
Matchmaking, Steam Cloud, Steam Stats
64 achievements
Tile-based map generation tool
Powered by Source and Steam

More info here.