Valve will release Alien Swarm - a four-player, co-operative, top-down shooter - on Steam for free today.
The PC game is an entirely new version of an old mod for Unreal Tournament 2004. The developers, Black Cat Games, are now employed by Valve, and have remade their game in Valve's Source engine, as RPS revealed last week.
The free Steam download includes the complete game code and mod tools for you to have your wicked way with.
You can watch a trailer at the Alien Swarm site. Looks tasty, and frankly, for nothing, it would be rude not to. The game unlocks tonight.