Epic has UT. id has everything. 3D Realms have Prey and Duke Nukem 3D for which the source code is out there and people need the files for running it on Linux.
Who's going to drive a truck around selling nothing
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.
Ignorant, big-mouthed kids