Then
Dungeon Master was a computer game released about 1987 by FTL (Faster Than Light) technologies. It was originally released for the Amiga and quickly ported to the Atari ST. Since then a PC port was made, a sequel was made (Chaos Strikes Back, Amiga only) and a Dungeon Master II was created for the PC.

Dungeon Master was the first roleplaying game that used realtime instead of a turn-based engine. This revolutionized the computer roleplaying game market. For the time the graphics were very good as well and the atmosphere it created was unparalled.

Now
FTL has long since gone out of business, they were bought by Interplay (or at least they created Dungeon Master II). The fan community has created several projects since then. Those include the following:

DMute: A program to edit save games and dungeons in Dungeon master.
("http://www.mrao.cam.ac.uk/~gg215/dm/index.html)

RTC: A clone of dungeon master not yet complete which will allow for more Dungeon Master type dungeons to be created with more control over them.
(http://www.mrao.cam.ac.uk/~gg215/dm/RTC.html)

The DM/CSB Encyclopedia is a complete resource for DM information.
(http://dmweb.free.fr/DMCSBHome.htm)

DM Message Board is a place for current discussion about Dungeon Master projects and issues.
(http://pub17.ezboard.com/biansmessageboard69920)