The different between this game and left 4 Dead is that Killing floor is a strickly non narrative multi player game like Team Fortress. You simply join a server with other people and you go from map to map trying to survive. Imagine trying to move through a map in Left 4 Dead in a never ending crescendo mode, and you have Killing Floor.
Released in May 2009, Killing floor's origins begin in the Unreal mod community. Originally a free mod in 2005 using the Unreal 2.5 engine, the game became an official release years after it's creation.
There are six classes to pick from in the game:
You can level each class up to level five for added bonuses and discounts on weapons. This experience component is a nice change from Left 4 Dead, where everyone is the same and there are no progression rewards for playing a certain way repeatedly.
As a game to play online, it's a lot of fun. The game lets you know how many monsters are left in a wave, and you can plan accordingly. After the monsters are cleared, there is an ammo shop you can go to replenish ammo and buy new weapons. The monsters get tougher after every wave, coming in with more numbers at a time and also with more special monster types. For example, there is a zombie that walks around screming, stunning the players with the echo of her voice. The game has a boomer, but it's not as good as Left 4 Dead's boomer.
If you're looking for something fun to play this summer, try Killing Floor. It's cheap and it has a strong community of players so you'll always find a game. Right now the group I'm playing with is so awful that we'll probably be playing for quite a while, just to beat the game on normal. It's different enough that it doesn't feel like a cheap imitation of Left 4 Dead, and the class progression is a nice perk to have in a multi player game.