ClassicDOOM.com -- Walkthroughs and guides, game comparisons, passwords, links and more, for game-console and computer-based Doom games
Crap Shoot -- A humorous review of the Novelizations on PC Gamer
Doom remix project: The Dark Side of Phobos -- 22 (2 CDs) fanmade remixes of original Doom soundtrack, download available including high quality CD covers.
Doomsday HQ - Home of jDoom, jHexen and jHeretic! -- Doom for the 21st century! Play Doom, Heretic and Hexen with OpenGL and Direct3D support, high resolutions and up to 32 players in multiplayer! There are also MD2 models (3D models) available!

Doomsday works kind of like ZDoom, Doom Legacy and Skulltag.
Doom Wiki -- A Wiki site for the Doom series.
DoomWorld -- The latest Doom news, levels, Total Conversions (TC's), ports and more.
JDoom -- Arguably the best revisited Doom engine. Take your original WAD files from Doom 1, 2 etc. and run them on this D3D/OpenGL and A3D enabled engine.
Matt Chat 53 -- Video interview with John Romero about the development of DOOM
OC ReMix Game Profile -- Fan remixes of music from DOOM, including the album "The Dark Side of Phobos".
PlanetDoom -- Lots of info, screens, wallpapers, fan-art, FAQs and much much more!
S&F Prod.'s Doom Page -- Here you'll find a Duke in Doom add-on and more.
Video review of Atari Jaguar games (WARNING: Language) -- The Angry Video Game Nerd, James Rolfe, reviews Atari Jaguar games, including Doom on Jaguar.
Video review of the system (WARNING: Language) -- The Angry Video Game Nerd, James Rolfe, reviews the Sega 32X and some games, including Doom for 32X.