online The game called MUD was

Found at: gopher.endangeredsoft.org:70/games/mud

Multi-User Dungeons, or MUDs, were some of the first multiplayer
online games. The game called MUD was the invention of Roy Trubshaw
and Richard Bartle, but a bewildering variety of clones emerged over
time: AberMUD, DikuMUD, MOOs, MUSHes, and more. In addition to
gathering documentation on MUD history, the ESS plans to host active
AberMUD was the first open source MUD. A number of versions were
released over the years, but it was primarily developed by Alan
Cox, a name which is well known amongst the open source
community. Among other things he was the maintainer of the 2.2 branch
of the Linux kernel.

AberMUD History(via a forum post by Alan Cox)

AberMUD Files

Off site links:

A Classification of MUDs, by Martin Keegan