TBAMUD  [codebase]
The maintained continuation of CircleMUD, and the usual answer to "I want to
start a Diku-style MUD today".
https://tbamud.com/

Games we have identified as running this codebase
  5 listed, 0 archived
  /games?codebase=tbaMUD
  Measured in their handshakes: MSDP, MSSP, MXP

tbaMUD — The Builder Academy MUD — picked up CircleMUD where its own development
stopped and has kept it building, patched and documented since. If somebody is
starting a levels-and-combat MUD from a known-good base rather than from their
own fork, this is the base.

It carries the Circle codebase's virtues forward: readable C, a documented area
file format, and a builder's manual that assumes you have not done this before.
The Builder Academy itself runs as a teaching game where people learn to build
on it.

WHAT IT LOOKS LIKE FROM OUTSIDE

MSSP, answered on request. A measured zero on the game we probed — which on this
site is a filled cell rather than an empty one: we got in, we read the number,
and the number was nought. That is a different fact from a game we could not
count, and the two are never merged here.

See also
  CircleMUD — /reference/codebases/circlemud
  DikuMUD — /reference/codebases/dikumud