This Monday, July 19, Steam will be releasing Alien Swarm for the low, low price of free. Once upon a time Valve hired two talented modders behind the top-down co-op mod, Alien Swarm. Since then they have been working on the Left 4 Dead series, but managed to hang on to the side project of bringing Alien Swarm to Source.

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. 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.

* 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