BitOwl
BitOwl.com/sde/

Simple Doom Editor:

Simple Doom Editor is a GPL lump editor for all Doom engine games (including Heretic, Hexen, and Strife).  Simple Doom Editor will be cross platform with its primary focus being Linux users.

Open Source

SourceForge.net SVN Access

The Simple Doom Editor source code can be found on SourceForge.net.  If you have knowledge in C++ programming you can join the Simple Doom Editor project by sending an email to devs@bitowl.com.