Doom News
DoomWorld - The definitive source for Doom news, information, and development.

New Doom - Your resource for everything to do with Doom, Doom II, and the new Doom!

Doom Center - The Gamer's Choice.

Source Ports
ZDoom - The most popular Doom sourceport. Also the base of ZDoomGL.

EDGE - EDGE (previously DosDoom) official homepage.

NTDoom - NTDoom homepage. Base of WDMP.

Team TNT - Creators of Boom, as well as many different Doom TCs/PCs (Including TNT and Plutonia, later released as Final Doom).