DOOM2. It's opensource and the FreeDoom Project develops an IWAD
(doom2.wad). That file + a Doom2 port (the most compatible and bug-free IMHO
is prBoom) + a multi-user WAD = open-source deathmatch! Yay!!!
(FreeDoom and prBoom are both available at SourceForge.net, as are many
other good game projects, BTW.)
Quake2 is open-source now, too, and I think that I found a Matrix deathmatch
TC, but it's incomplete and I'm not sure you can find it on the Net. Try
Quake2.com first.
Aaron
(e-mail address removed)
www.neowulf.net (soon)
(e-mail address removed) (soon)
P.S.
As always, I try to keep extensive back-ups of the source and apps I find,
so if you can't find something I mention, let me know and maybe we can work
out sending you a CD-RW in the mail or something.