I thought I'd link to this free Steam game coming out soon. It was originally a UT mod but Valve swooped them up and they made a second game for the Source engine.
Did I mention it's free? Oh yeah I did.
http://store.steampowered.com/app/630/"Along with the game get the complete code base for Alien Swarm that features updates to the Source engine as well as the SDK. Alien Swarm adds 3rd person camera, depth of field, improved dynamic shadows and a wide variety of gameplay additions to the Source engine."
Win.
Good stuff.