DeuTex
WAD composer tool, which can extract all types of data from WADs, and build WADs from a collection of resources.
DMapEdit
Level editor for DOS. Includes download, screenshots and a tutorial.
Enjay's little place of Doom
Personal site with a guide to sprite editing and using Dehacked.
glBSP
New standard for Doom nodes data needed for OpenGL rendering, and the node building tool to produce this data.
SBSoftware
Free download, help, and online registration for DeePsea, a fully featured Doom editor for Windows.
Why
Discontinued editor for Linux and Windows, which supports python scripting.
XWE
eXtendable Wad Editor, which edits levels and associated resources for Doom levels and a range of other games.
Yadex
A level editor for Unix and Linux systems which supports the Doom games, the Doom betas, and Heretic, Hexen and Strife too.
ZenNode
Fast nodes and reject builder, for Windows and Linux.