Yesterday the lovely folks at Valve gave us Alien Swarm for FREE.

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. Along with the game get the complete code base for Alien Swarm that features updates to the Source engine as well as the SDK.

Having put a couple of hours into this game last night it feels like a over head version of Left 4 Dead. It's quicker paced and can be very challengeing. There is only one campgain implemented currently, but with a SDK released with the game there is sure to be no shortage of third party content.