Tripwire Interactive's cooperative zombie shooter Killing Floor shows no signs of declining in popularity, if my Friend's List on Steam is anything to go by. It's still a solid, really enjoyable co-op experience that's been supported with plenty of paid DLC and free updates since its launch in 2009. If you're still on the fence about playing Killing Floor, then perhaps a free weekend might be of help.
Free weekends on Steam have convinced me to buy plenty of games and if you can find some friends to play with, I heartily recommend Killing Floor; there's plenty of maps to choose from and plenty of incentive to progress up the ranking system. You're not 100% dependent on teammates as you are in Left 4 Dead, but it's still best to play with buddies.
Tripwire has also teased a cryptic image on its Steam community page, which looks like it might be revealed before Thursday. What could it be? A stack of new weapons or enemies? I'm hoping for another Christmas bonanza like last year. 'tis the season to be jolly indeed!