Don't know why the links says pre-purchase.
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.
Sounds good and looks like a lot of fun. Interesting to see a non first-person game coming out of Valve. It's also great to see Valve hiring modders and releasing the work they do for free.