Szevvy's little corner of WWW - http://student.ci.qut.edu.au/~n4738667/ - Szevvy made the amazing Wombat Game Tools which can do a lot of editing stuff with Duke Nukem, so check it out.
The Duke Nukem 3D Repository - http://www.bur.st/~duke3d/ - A site that includes a rather cynical take on the first two Duke games.