UberMUD
An early MUD server where all the game's rules lived in a programming language inside the world rather than in the server's source.
Marcus Ranum wrote UberMUD around 1990, in the same wave as TinyMUD. The idea was radical for its day: the server knew next to nothing about games, and everything — rooms, commands, what happened when someone picked something up — was expressed in its own language, U, stored in the database. Changing the rules meant doing it in the world, not in a compiler.
UberMUD never spread the way TinyMUD's descendants did, but the notion of an empty, programmable core returned in MOO and became a hit there. Ranum himself became far better known for something else entirely: firewalls, where he was among the pioneers.