LambdaMOO

LambdaMOO is an online community[1] of the variety called a MOO. It is the oldest MOO today. https://en.m.wikipedia.org/wiki/LambdaMOO

There was a kind of successor to LambdaMOO, called ColdMUD, that had a far cleaner object model. Greg Hudson took the ideas of MOO and made a language with similar prototype basis but with a more consistent syntax and object system. I've archived the sources that on my github (I'm even in the CREDITS file despite contributing... not much... ) https://github.com/rdaum/coldmud

2026: MegaMOO is a comprehensive pure Python world-building engine modeled on the LambdaMOO paradigm: data and behavior living on objects in-game; live creation and hot world editing--no reload and no restart; an auth and permission system that lets you give builders true authoring power, sandboxed by ownership; a suite of authoring commands that lets you build right out of the box without writing a single line of code, but also puts the power of in-game, on the fly Python scripting in your hands.