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Alien Swarm is now available via Steam.

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. Along with the game get the complete code base for Alien Swarm that features updates to the Source engine as well as the SDK.

Alien Swarm is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community