“Alien Swarm is a game and Source SDK release designers at Valve who were hired from the Steam Mod community.
Available for free, the game thrusts players into an epic bug hunt featuring a blend of co-op play and squad-level tactics. Form a squad with you friends of four distinct IAF Marine classes. Plan your attack using an unlockable battery of weapons with many loadout configurations against a large 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.”
http://store.steampowered.com/app/630/
System Requirements
OS: Windows® 7 / Vista / Vista64 / XP
Processor: Pentium 4 3.0GHz
Memory: 1 GB for XP / 2GB for Vista
Graphics: DirectX 9 compatible video card with 128 MB, Shader model 2.0. ATI X800, NVidia 6600 or better
Hard Drive: At least 2.5 GB of free space
Sound: DirectX 9.0c compatible sound card