Q3Radiant is the name of the
editor application that was used by
Id Software to create the levels for their popular 1999 game,
Quake III Arena. Id have shrewdly made this powerful tool available to the public (ftp.idsoftware.com), thereby enabling
Win9x and
NT users to create their own levels (and
mods) for
Quake III Arena.
Q3R was developed from the extension of the Quake 2 editor, QERadiant. It allows level geometry (including curved surfaces) to be edited with a high degree of flexibility, and also allows the placement of entities and the designation of targets and triggers to help the mapper implement a wide variety of special features and obstacles within their arenas. Using such a heavy duty editor can be daunting at first*, especially in terms of remembering the many shortcut keys.
Q3R can be extended with plugins (such as one called 'Curry' which allows you to preview shader effects), and can be set up to support other Quake III-engined games such as Return to Castle Wolfenstein (or at least, this is promised in the near future).
WWWWolf reminds me that there is a related cross-platform version of the editor called GtkRadiant.
*Especially if, like me, you are too damn stupid to even be able to construct a DooM level without forgetting which direction the vertices are supposed to go every five minutes...