Doom Builder

Doom Builder is a Doom level editor with full support for DOOM/DOOM2, HERETIC, HEXEN, ZDoom and Doom Legacy editing. It is being developed by Pascal vd Heiden 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 which lets you browse and edit your textures, and align them, in realtime.