http://www.quakeone.com/qadapter
It's fairly good. RickyT23 has made some masterpieces with it and is a big advocate of it and as far as I know MDave made the Kurok maps with it.
I'd prefer a REAL improved map editor that didn't need "hacky" gear to make it work natively with Quake.
That being said, I made the package for Quake Expo 2006 as an all-out assault on all the questions of newbies getting setup to map for Quake and was tired of watching the Three Stooges like "clear as mud" instructions that followed (what map compiler? Where do I put it? How to I set it up? What folder? What do I use for textures? How do I configure it? What settings on the setup page? What about x, y, z ...).
It has been largely successful in that and has been downloaded a very large number of times (10,000 +/-).
That being said, the BSP map editor and Quark and even Worldcraft 1.6 are commonly used to make high quality maps today.
Links for about 75% of the most important resources in Quake modding:
http://www.quakeone.com/navigator
The night is young. How else can I annoy the world before sunsrise?

Inquisitive minds want to know ! And if they don't -- well like that ever has stopped me before ..