Now hold on to that image, and imagine if the Suffering...

...Doom 3...

...and Left 4 Dead...

... had a child... in England.
It would most likely look something like this:



Its called Killing Floor, and it just came to steam for $19.99 (USD).
Trailer
Basically, you and 5 other teammates fight off waves of these biological hell spawn and earn money for each kill. after each wave a weapons shop opens up for a set amount of time. Weapons vary from shotguns to chainsaws to pistols to flamethrowers ect ect. awesome graphics, awesome sounds, and awesome baddies.
originally it was a unreal tourny 2004 mod, but now is a stand alone game... and seeing as the sdk came out at the same time as the game, that means custom levels.