Recently on Steam, Killing Floor was updated with some new content along with allowing everyone to download and play free for the entire weekend (which is something that happens on Steam quite frequently with other games).
Killing Floor is an UT2K4 mod made by Tripwire Interactive, creators of Source Mod Red Orchestra.
When Killing Floor was first released, I figured it was just a Left 4 Dead clone and even though I was thinking about buying it, I ultimately decided not to (as I found Left 4 Dead to be very repetitive and didn't want to go through that again).
I decided to pick it up during the free weekend, and well, I haven't stopped playing since. I bought it for $14.99 21 minutes before the free weekend was over, as I did not want to interrupt my game play.
I have logged more than 30+ hours playing this game in the last 3 days. Seriously.
I will create a more in-depth review later, but my opinion is BUY BUY BUY.