Doom Builder

Doom Builder is a popular new editor for Doom with support for both vertex and linedef based editing styles. It also allows you to edit light levels, sector heights, and textures easily in a realtime 3d mode. It also features support for many Doom ports, including ZDoom, Eternity, jDoom, Boom, and Doom Legacy. It is being developed by Pascal vd Heiden and is free software, licensed under the terms of the GNU General Public License.

Official Site

http://doombuilder.com/index.php