Other than Unreal Tournament, any games on GOG have a active multiplayer community? I was interested in Vampire the Masquerade Redemption, but it seems it's multiplayer both doesn't work (not without the use of Hamachi or third party program) and is rather sparce in population.
Any ideas? :)

Possibly Sacred if youre looking for co-op multiplayer, though I'm not sure how many people still play it. That's all I can really think of, sorry.

That's the bane of old games, people just move on.
UT2k4 is the only one I can think of, and that I figure is because UT3 didn't fill everyone's expectations.

UT an UT2k4 both have semi-active ones.

Duke Nukem has a somewhat active multiplayer community. As well as a modding one. Maybe that's not so obvious. Does your question include games that didn't originally have an ingame browser?

HOMM 3 if you use game ranger.

Far Cry is okay...

Does pain killer have a good online community