MOO  [codebase]
Object-oriented, edited entirely from inside, and as much a research and
teaching platform as a game engine.
https://www.ipomoea.org/moo/

Games we have identified as running this codebase
  None yet. That is a statement about what we have measured, not about what
  exists — a game we have not reached, or whose codebase we could not read, is
  not counted here.

MOO — MUD, Object-Oriented — takes the "the world edits itself" idea further
than anything else in the hobby. LambdaMOO, the original server, ships a small C
core and a database; essentially everything a user experiences is written in the
MOO language, inside the running database, by the people using it. There is no
source file for a room.

That property gave MOOs a life outside games. Through the nineties they were
used for teaching, conferencing and research — Diversity University, BioMOO,
Jay's House — and the technical literature about MOO is disproportionately
academic for a codebase in this space.

Deployment today is small but genuinely non-zero, and the servers that remain
have often been running continuously for decades.

WHAT IT LOOKS LIKE FROM OUTSIDE

No MSSP, and no WHO we could parse on the game we measured. What it did have was
a sentence in its connect screen reading "one of three players are active" —
which is where the spelled-out number reader in this crawler comes from. A
digits-only parser sees no count there at all, and would have reported that game
as unknown for ever.

See also
  MUSH, MUD, MUCK, MOO — what the words mean — /reference/mush-mud-muck-moo
  MUCK — /reference/codebases/muck