I want to give a shout out for ezQuake as well. It's a GPLv2 QuakeWorld engine. Last time I checked, admittedly three or so years ago, there were still people playing QuakeWorld, especially in Europe. A lot of duel and 2v2. Many of the servers run KTX server-side mod which is also GPL
http://sourceforge.net/projects/ezquake/Community:
http://quakeworld.nuI'll be reinstalling again soon, curious to see if any of the old regulars are still around.
Biggest problem with QuakeWorld is that it can be hard to find competitors that aren't practically professional-tier. It gets frustrating losing 50 to -2 over and over :DDD