Valve and Steam have showed their love to PC Gamers once again by releasing Alien Swam for free. It is an improvement upon the UT 2004 mod and uses the Source engine. Full source code is available as well.
My friends and I have been playing this and I must say for a free game it really is awesome. Very sickening to me that the team working on Portal 2 and Left 4 Dead 2 did Alien Swarm in their 'free' time. Proves that with the right talent and tools you can pretty much make anything you want.