codebase
tbaMUD
The maintained continuation of CircleMUD, and the usual answer to "I want to start a Diku-style MUD today".
Games running it
5 listed · 0 archived · measured, not declared
Counted from the catalogue on this request, over the same filter the link above carries — so this number and that listing are one query and cannot drift apart.
offered 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.