michaelleung: Or just pay $10 for UT2K4 on GOG and download the mod Killing Floor as opposed to the game that costs $15.
I must confess I didn't give Killing Floor so much as a glance until recently. I've lost interest in (and patience with) mods over the past few years, parallel to my waning interest in FPS generally. The ones I enjoy tend either to die or to make a beeline for Idiocracy Mode - and whatever I first read about KF led me to believe the experience involved arenas somewhere between Zeno Clash's challenge arenas and Q3A's duel maps. Not that I find fault with that sort of thing, but I've closed the door on it. Were it not for the free weekend a while back I probably still wouldn't know much about KF.
Having said that, the thread is about multiplayer games, and as near as I can tell the UT version of Killing Floor is no longer regularly played online. If there's a version other than 2.5 which has retained a functioning community - or even an official web presence - I sure can't find it.
And if I had 5 friends who would indulge my obscure gaming whims on a nightly basis, I would probably be playing Giants or Fallout Tactics right now.