The father of it all, 3dRealms is the place to find all 3drealm games (DukeNukem tm, Shadow Warrior tm etc.,). Great place to find new games.
http://megahertz.njit.edu/~mxs1136/files.htm
tons of deathmatch maps and a couple of single players.
http://www.softwaresource.com/gsadd.htm
links for a whole ton of other Duke sites (pathetic, a link for a links site)
http://netrover.com/~benroff/duke.html
5 or six great maps
http://www.wizard.net/~greg/duke.html
tons of single player maps, lots of aliens and tons of guts, this site is dedicated to single player maps, no deathmatchs allowed
where I download most of my games. Its a great site, but takes a while to download sometimes. Thousands of FREE games!