A couple years ago Valve hired the folks who made the Alien Swarm mod to work on their various other projects. During that time they slapped together a Source version of the game, with brand new Source engine enhancements to be released alongside. And it also comes with Steamworks integration. And it's free. And you can download it on the 19th.
That's pretty cool!
Store Page: (download/trailer/screens/details) 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. 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.
- Tactical, 4 player co-op action game with a top-down perspective
- Complete game code and mod tools
- Unlock persistent items by gaining levels
- Over 40 weapons and equipment with countless loadout configurations
- 4 unique classes and 8 unique characters
- Matchmaking, Steam Cloud, Steam Stats
- 64 achievements
- Tile-based map generation tool
- Powered by Source and Steam