You can find extensions at the plugins page. See the screenshots page for a quick look at this editor or see the downloads page for the real deal.
Doom Builder introduced the 3D editing mode in the Doom community and is still the leading editor for Doom levels today. This editor is highly extendible for the different game engines of the Doom community.
Doom Builder is an advanced 3D map editor for Doom and games based on the Doom engine, such as Heretic, Hexen and Strife.
