Well, I do belive some server might be overexagerating about being number one. According to Game-Monitor, Undead house Is the number one server.
http://www.game-monitor.com/search.php?game=zombie_master2

Ask me For password

The Server is unlocked And in about 30 Minutes the events will start up so come in quickly if you want your slots,

I Might lock it when everyone is inside

Corpse: The Epidemic
http://www.freewebs.com/cyclone07/zombiemod.htm
Release date - No clue

RE: Surivial Horror -
http://www.evilmod.de
Release date - Dont know

No More Room In Hell-
http://www.nomoreroominhell.com/?p=files
Release date - Clueless

Zombie Panic :Source - http://www.zombiepanic.org/site/index.php
Release date - Already out

BrainBread Source- http://www.ironoak.ch/BBS/
Expected Release date - Still in Creation

Left 4 Dead - http://www.l4d.com/
Expected Release date : June 2008- Late 2008 Summer

The Survivors - http://ts.hlgaming.com/
Expected Realse Date : Already out