I just got done messing around with a co-op mod some people made. Here's a link with some links to the files:
http://forum.unitedgamerscoalition.c...y;threadid=694
In use you only need two files:
x_coopmenu.pk3
z_coop01.pk3
The files in CO-OP LOADERS are just windows Shortcuts for starting up.
There are instructions on how to join a coop game within the menu system. The only way I've been able to connect is by knowing the host's IP address.
Key notes:
You only play one map at a time. I'm not sure if the server will auto switch. The maps are custom co-op versions of the singleplayer. There's one spawn point, so once you spawn, run or else you'll telefrag someone, or worse, crash the server (haven't seen that happen yet though).
There are currently only a handful of co-op maps, but apparently you can make your own with instructions included with the 3 downloads.