A full-blown port of the M1 scenario, Devil in a Blue Dress. I won't hype this too much, since it's my baby, but it contains 21 solo levels (painstakingly debugged), almost 7 megabytes of original artwork, and 2 megabytes of original sounds. If you've played the original, you'll find all-new puzzles, secrets, and enhancements. If you haven't, this should keep you busy for almost as long as Bungie's M2 scenario.
The first release of the Marathon Mapmakers Guild, Devil contains 20 levels, along with cool original art, an entirely new storyline, and some nice twists. This version is intended only for folks who haven't played Devil yet. It fixes or works around all known bugs, and beefs up the documentation a little. This is the final release of Devil.