I love YaBB 1G - SP1!
It's by RocketGuy and it's pretty cool. It's a cooperative versus the monsters mod, not versus other players.
It's not the usual. It's pretty cool and can even do cooperative on non-cooperative maps due to an engine modification and some QuakeC.
It even has a flashlight. Requires custom maps.
Right now, there is 1 server running the mod and this is a regular Quake mod (JoeQuake/QRack, DarkPlaces, etc.)
The server has map voting and even has a map with screenshots/teleporters of all the available maps on the server:
If you like cooperative play:
http://www.quakeone.com/rquake
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