DMute is a Windows based editor for both the PC, Atari ST and Amiga versions of Dungeon Master and its sequel Chaos Strikes Back, developed by Georges Gilbert (george@ragingmole.com). It allows the creation of completely new dungeons by modifying an existing dungeon.
It can also edit saved games (champions' stats, skills, items, ...). The latest version can also save dungeons in Return to Chaos format.
You can visit the DMute web site.
It was the first editor released for Windows at a time where the full dungeon file format was not yet decoded, which explains why it is not a 'full' editor: you cannot create a dungeon from scratch. You must load an existing dungeon and then you can only change and move existing things but you cannot create new items (armor, weapons, etc.), add/remove maps in the dungeon or change their dimensions.
Moreover, you can only load a predefined and limited set of dungeon and saved game files. Other files (like French and German versions of dungeons and saved game files) are not supported and may even cause the program to crash.