CIRCLEMUD  [codebase]
The DikuMUD derivative that was documented well enough to teach with. Enormously
forked; still running.
https://www.circlemud.org/

Games we have identified as running this codebase
  47 listed, 0 archived
  /games?codebase=CircleMUD
  Measured in their handshakes: MSSP, MSDP, MXP, MCCP, GMCP, EOR

CircleMUD is a DikuMUD derivative whose distinguishing feature was never a game
mechanic: it was the documentation. Jeremy Elson's release was clean, commented
and accompanied by a coding guide, and as a result it became the codebase people
learned C on, ran a MUD from, and forked without needing to reverse-engineer
anything first.

The consequence is that a very large number of running games are Circle
derivatives several generations removed, often without the word appearing
anywhere a player would see it.

Development of Circle proper is long finished; tbaMUD is the continuation, and a
Circle game being maintained today is usually being maintained as tbaMUD.

WHAT IT LOOKS LIKE FROM OUTSIDE

MSSP, answered on request. No WHO at the login screen — the Diku family
generally does not offer one, which is why a directory that only reads login
screens sees nothing here.

See also
  DikuMUD — /reference/codebases/dikumud
  tbaMUD — /reference/codebases/tbamud
  ROM — /reference/codebases/rom