The Forge (planetquake.com/worldcraft)
The Forge was one of the first sites online tailored to Quake level editing, specifically mapping using pre-Half-Life Worldcraft. The site was started by a fellow named Chris "autolycus" Bokitch, who'd later go on to start the Half-Life/Valve Editing Resource Center site and get hired by Valve in the process. Given how much it influenced VDU early on, and given the state of disarray it's in elsewhere, I figured it deserved better.
Bengt Jardrup is an old-school Quake mapper and tool developer best known for his work on Enhanced WinQuake and GLQuake. His site is small, but features some curiosities, including an archived QMap thread, an extensive text file on troubleshooting and map innards, and some older compile tools with features not found in modern sets.