This brilliant classic game tool looks very promising. It's plans are to be able to export/import the graphics, sounds, music, and other important files from all common classic DOS games. It even plans to be able to view the levels of the games. Right now it's in beta, but viewing and extracting graphics from Duke is almost completely bug-free, as are the routines for many of the other games. This is well worth your time downloading.
This is really two different editors that are completely different. One is a flexible Windows editor (v3.0) with only basic features. The other (v2.0) is an inflexible DOS editor, however it has a lot of neat stuff included. Version 2.0 has a number of fanmade level sets included. Visit the site for more details here.