Alien Swarm
Okay, so on Friday, Valve suddenly came out and said “Hey guys! Guess what? We have a new game coming out on Monday, and it’s going to be free!”
And so I (and everyone else) was all like “ASDFBURGER”
Being a fan of Valve, (and free stuff,) I looked into this. Alien Swarm is a Source-ified version of a mod of the same title that came out in 2004 for UT2K4. The mod team was hired by Valve, and in between working on Left 4 Dead and Portal 2, the team was quietly working on a new version of their mod. And they’ve finally released it.
I played it a bunch yesterday, and the game is epic awesome, if a bit short. But that’s what they released the SDK for.