Doom Builder

From ZDoom Wiki

Doom Builder is a Doom level editor with full support for ZDoom editing. It is being developed by Pascal vd Heiden (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 realtime.

If you are new to pwad editing or just want review certain items, then read Doom Builder: An Illustrated Guide. It is an invaluable read for anyone using CodeImp's editor.
You can find the guidebook on Dr. Sleep's website where it is available for download in Adobe Acrobat PDF and Microsoft Word Document form.

See also the Doom Builder ACS configuration.

The last version of Doom Builder is 1.68. Doom Builder 2, a total rewrite, is currently in development.