Yarrr, here be dragons. I mean… Here are some external links to other sites containing useful information for editing the third Doom game:
http://www.iddevnet.com - id’s developer network, currently covering the Doom 3 SDK.
http://www.doom3world.org - long-running Doom 3 site covering just about everything (hence the name), run by BNA!.
http://blaze.spicelab.net/doom3/ - collection of tutorials aimed at beginners (some singleplayer focus).
http://www.doom3maps.de - German Doom 3 mapping site (thanks burzum).
http://www.qeradiant.com/wikifaq/index.php?DOOM3%20Level%20Editing - radiant wiki FAQ thing (thanks zig-zag).