it's a pretty good mobar

Killing Floor started out as a mod for 'Unreal Tournament 2004' in 2005. Then standalone version was released in 2005 and has had continued success through it's frequent updates and Downloadable content releases.

The game is labeled as "Co-Op Survival Horror", and while that may at first sound like an oxymoron, don't dismiss the claim.
You and up to 5 other teammates fight through multiple waves of different brands of mutated specimens, all looking to eat you alive. Each kill on a mutant rewards you with a certain amount of money and surviving the round nets you an extra bonus. Why do you need this money? To buy the bigger guns, of course. This game gets progressively more difficult with each successive waves, with later waves introducing super-strong 'mini-boss' like characters.

Once you reach the final wave, you must face "The Patriarch." he will face you and your teammates alone at first, occasionally bringing in some smaller weaker mutants to distract you.

"But that means it's 6 versus 1? How hard could that possibly be??"
Very.

Because here is what The Patriarch has in his arsenal:

--Invisibility cloak
--Super Speed
--Super Strength
--Syringes of a strange formula that allows him to regenerate health.
--Chain gun
--Rocket Launcher arm

Yes, that last one says "Rocket Launcher arm."

If you can bring down the Patriarch, you will win the round and move on to the next location, rinse and repeat. The variety of weapons, along with the perk system and user created content support really add to the game's replayability value, and with the second installment coming out in the near future, this one is great to pick up to familiarize yourself with the world.

This reimagining of the 2001 Hit First Person Shooter retains everything you loved about the original, while fixing the things you hated. This game is also garnering a very large competitive scene, which is worth watching.