QuArK - Quake Army Knife
Very cool and reliable map editor for Quake-engine games - besides the Quake series it supports Half-Life, Soldier of Fortune I and II, Return to Castle Wolfenstein and many others.
It can also edit models, import sounds and textures, create PAK files and compile maps.
I used it to create my maps for Quake and Return to Castle Wolfenstein.
More information can be found on the official page of QuArK.