GtkRadiant under GPL
gnarlin writes "GtkRadiant is now available under GPL license, along with some Quake II tools which never got a chance to be put under GPL before. Head over to the qeradiant website for all the details. GtkRadiant is a level editor developed by id Software and Loki Software. It is used to create maps for a number of computer games."
For what it's worth, GtkRadiant is not exactly the same level editor as the one that was used by ID during the development of Quake: rather, it's a heavily modified version of QERadiant (Quake II's level editor) and Q3Radiant (Quake III Arena's level editor), which were used by ID.