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.

ToTac says

10/10 - Agree (1) Disagree (1)

Alien Swarm brought to me so much hours of cooperative fun with a nearly perfect gameplay and gamedesign, that it actually deserves to be sold as a real game. Since it was not further developed, which I totally can not understand, I am afraid it wont be as succesful as full retail games, that however have a worse gameplay (Alien Breed e.g.).