Of the two most popular, one that is comitted to hosting Killing Floor 2.50 is Jeuxlinux.fr. The other server that hosts KF 2.52 is Miasma UT2004. There is a big difference between the servers. Jeuxlinux.fr has their server assigned to "Experience" mode, while Miasma UT2004 have theirs in "Skilled" mode. And the collection of maps is equally divergent. Jeuxlinux.fr has at least thirty plus maps. Among them are the popular KF-Downtown, KF-Colloseum (by KINGshinobi), KF-Mall, KF-Arcade-Shopping-Center-Final, and many more. Miasma UT2004 administrated by Raj has a few collections as well, many of which are new maps designed by mappers such as Vanico, Hicks, Audrey-Rafael, Snipe34, and Mahloeeh500.
So if you are ever in the mood for KF 2.50 gameplay, just head over to Jeuxlinux.fr. And if you're into more challenging maps, then head over to Miasma UT2004.
Sceernshots below - Jeuxlinux maps: