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.
Here's a video of the game: