The Prospect Vector page has the initial release of the Tread Quake-engine level editor (not to be confused with the long-abandoned, but once formidable Thred). Tread features full Quake, Quake II, and Hexen II support, with Half-Life support on the way. There was a glitch with the very first upload, so if you happened to be a really early adopter, you should head back over and get the proper version.