http://store.steampowered.com/app/630/
Alien Swarm is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community. 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.
It started as a mod for Unreal Tournament 2004, which I played a bit and enjoyed, and this both looks and should presumably play better so it could be pretty good!
In case you missed it, there's a launch trailer on both the website and Steam page.
