Gtk Radiant is a program that can creat .bsp levels for the following games, I think:
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Quake - Editing Info For Quake 1
Quake 2 - Editing Info For Quake 2
Quake 3 - Editing Info For Quake 3
Return To Castle Wolfenstein - Editing Info For Return To Castle Wolfenstein
Jedi Knight II: Jedi Outcast - Editing Info For Jedi Knight II: Jedi Outcast
Medal of Honor: Allied Assault - Editing Info For Medal of Honor: Allied Assault
Half-Life - Editing Info for Half-Life
Heretic 2 - Editing Info For Heretic 2
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I know it supports Halflife and Quake 1, 2, and 3 and Jedi Knights II. You go to the website which is: www.gtkradiant.com to search for it, or you can do it the easy way on go here: http://www.qeradiant.com/?data=editors/gtk and read what it says and then go to the downloads and download it.