The original Alien Swarm was an Unreal Tournament 2004 modification made by Black Cat Games. The same team was working on a Source version of their game, planned as a full retail release.
"Two years ago Valve hired the talented team behind the popular top down co-op mod Alien Swarm. Since then they have been busy working on the Left 4 Dead Series, and now Portal 2. However, we never forgot about Alien Swarm and the team has spent a lot of time bringing the game to Source in between their contributions to the other Valve projects."
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. 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."
As a true Valve game, Alien Swarm will support the full suite of features on Steam including achievements, Steam Cloud, matchmaking, stats tracking, and VAC for anti-cheat protection. It has not been announced for the Mac yet, however.
"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 adds 3rd person camera, depth of field, improved dynamic shadows and a wide variety of gameplay additions to the Source engine."
I've been playing through GREED right now, so this sounds awesome.