Coop in Quake 3?
I never cared for Quake 3, because it was an MP only game.
As a result, any mods that would have been made for it would have been completely ignored.
I was to engaged in Half-Life as well to probably really care.
I also didn't care much for Quake. I much preferred Duke Nukem 3D which had a lot of atmosphere and humour, and a bit more coherent and interactive of a game IMO. I never cared much for the Quake games...but hopefully Quake IV will be more up to my liking.
I did play Doom 6 years later MP in our computer lab....but we also played Quake MP which is a lot better.
Although a friend of mine got me playing Duke Nukem Coop with him...and we've just finished Episode 3 :D Just have the new Episode IV, which I actually never played before.