Valve will be releasing Alien Swarm for free today on Steam. Alien Swarm was originally created as a mod for Unreal Tournament 2004 by Black Cat. The version Valve is releasing today has been completely re-written as a stand alone game and according to the official announcement, "In addition to the game, Valve will also release the complete code base for Alien Swarm. This includes updates to the Source engine SDK and full Steamworks integration. If you’ve ever thought about developing a mod on the Source engine with Steamworks, this release provides more insight and examples for using Steamworks in game production."
The game uses Valve's Source engine and will include full steam support. The game is a co-op, top-down shooter with one primary focus, blowing up aliens. In the game you can play as one of eight different characters in one of four different classes using up to 40 different types of weapons and equipment. Players will gain experience and level up as they play unlocking different items along the way.
The team that created the original Alien Swarm mod was hired by Valve back in 2008. Since being hired, they have helped in the development of the Left 4 Dead series, Portal 2 and, in their spare time, they continued development of Alien Swarm.
Update: Valve has released Alien Swarm, go get it while it's hot!