Site dedicated to Doom... news and forums, etc..
Rex's Doom site where you can find his levels, fiction and tutorials..
Doom Source Port
Doom source port based on ZDoom...
Ismaele's site that contains his Doom maps and reviews of various maps.
Tormentor's Doom related site.
Mr_Rocket's Doom related site.
Doom maps, utilities, somefin about voxels and a few hosted sites as well...
A place where Doom level designers can post screenshots of their work-in-progress.
Karthik's site contains his Doom levels, art, movie info and much more...