PlanetQuake.com/lvl - Probably the best general q3 map review site in existence right now. Reasonably even-handed reviews that pretty much everything that's out there, good user comments/ratings, deep archives.
Quake 3 World BBS - Level Editing forum that's regularly visited by id software emplyees and experienced level designers. Probably the best place to look if you're having trouble with radiant/compiling/etc. Please search the archives thoroughly before asking a question.
Qeradiant.com - Quake radiant home page. kind of light of community stuff but has good plugins links, documentation, links to other places...
GameDesign.net - Rust Tons of mapping info for every game. Extremely good q2 documentation, ok q3 documentation (you'd be surprised how much you can get out of the manuals that come with radiant though). Good forums.
Qmap Level Design Forum - Community-driven mapping site frequented by experienced q1dm/q1sp/q3dm/etc mappers. The latest site in the QuakeMap legacy of dm mapping boards (Publius thinks).
XSReality.com Challenge us.com Gamepig.com - Competitive gaming news if you interested in how the "other side" plays. XSReality has LOTS of q3dm/team demos, gamepig focuses more on CTF than the other two (with the occasional q3ctf demo), but XSR/CHUS have more active forums. GamePig does q3ctf QTV every once in a while (yay).
Q3ctf.org - Descent (but good) q3ctf news/commentary site.
Templay.net - Fall Festival of Frags, teamplay.net's current q3ctf tournament. Does qtv (yay).
Online Gaming League - Small (nonexistent?) alliance ladder, very large q3ctf ladder.
The Demoplex - q2ctf/q3ctf centered demo site, has its own invitational tournament (with its share of controversy, but I won't get into that), does qtv (yay).
Alliance homepage - Alliance homepage, you can download Alliance CTF here, check files section for a direct link to latest patch.
Alliance News - Alliance News site run mainly by our very own Corey Feldman. Find your latest Alliance related news here.