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Worldcraft
From Quake Wiki
A graphical level editor for id Software's Quake, Quake2, and Hexen2 Worldcraft lets you make your own game maps quickly and easily. Worldcraft uses a technique called constructive solid geometry (CSG) to create maps. It was one of the only few Quake map editor for quite a while back in the day and has since developed into a Half-life/Half-life 2 only map editor.
Worldcraft's Quake related pages can be found archived here:
[1]web archive