There are a couple of KTX coop servers but it's a mod with different features so no good vanilla Quake coop:
http://www.quakeworld.nu/news/326/
Oh WTF a WARPSPASM server! How is that possible. :o
excoop is a good mod, just bugfixes and stuff.
http://www.quakeservers.net/quakeworld/servers/d=coop/
These have truly gay mods running:
http://www.quakeservers.net/quake/servers/d=coop/
Honestly I think it would be a waste of time to setup. Not to mention that many recent maps won't be able to run. And that NetQuake sucks as network protocol whereas QuakeWorld rocks but not for Singleplayer/Coop.
Plus a quick coop game is easily hosted on a home connection.