Doom Builder
From ZDoom Wiki
Doom Builder is a Doom level editor with full support for ZDoom editing, including the "Game-in-Hexen" and "Game-in-UDMF" map formats. It is being developed by Pascal vd Heiden (alias CodeImp) and is free software, licensed under the terms of the GNU General Public License. Popular features include lots of customizability, prefabs, advanced drawing code, and a 3D mode that lets you browse and edit your levels in real time.
See also the Doom Builder ACS configuration.
The current version of Doom Builder is Doom Builder 2, a total rewrite. The last version of Doom Builder 1 is 1.68.
