From Quake Wiki

Worldcraft is a graphical level editor by Ben Morris which in its last unincorporated form had support for Quake, Quake2 and Hexen2.

Using a technique called constructive solid geometry (CSG), Worldcraft enabled the end user to craft their own levels for the aforementioned titles. It is easily one of the most popular level editors, if not the most, rivaled only by the Radiant series. Morris was later hired by Valve Software to continue the program under the name of Hammer in support of Half-Life and their following products.

Worldcraft's Quake related pages can be found archived here:

[1]web archive