From Quake Wiki
Func_Msgboard is the main level design forum/news site for Quake.
Quaddicted serves as both the major 'review' and archive of Quake levels. It also has an extensive archive of tools and files related to Quake and the modding community in general.
Quoth is a major mod for Quake that includes a suite of new enemies, weapons, and entity functionality that are invaluable for modern Quake level designers.