LambdaMOO is a shared virtual world system similar to a (or kind of) MUD. It was originally written by Stephen White (then at U Waterloo) and added to and maintained by Pavel Curtis (Xerox PARC) (and others) in C, and released in 1990, and was mainly popular through the early 90s (before the web took off).
It is novel in that the bulk of the world's behaviour is implemented in a virtual machine, and the world itself is stored in a shared persistent programmable object database.
The world is structured -- like other MUDs -- with some of the aspects of interactive fiction / adventure games, where users can move around, interact with objects, and interact with each other using a text-based interface. The focus was primarily social interaction and creativity, and it was/is great.
There were many LambdaMOO worlds, and the most famous in the 90s was LambdaMOO itself, which was a social experiment in shared virtual space. It is a text-based virtual world, where users can create objects, and program them in a prototype-based object-oriented language called "MOO".
The LambdaMOO server is still in use today (as is the LambdaMOO MOO tiself), and there are many active worlds running on it, but the community these days is much smaller and less active than it was in the early/mid-90s.
In its essence LambdaMOO is/was a live and interactive "social network" offering a somewhat richer social experience than what the web page-oriented systems of today offer. It's a shame that this line of evolution was mostly abandoned in the 90s.
The original LambdaMOO server is still available, but is showing its age, and the codebase is not easy to work with to add fundamentally new features like a modern database, or to support new network protocols, and most importantly, newer user interface modalities / presentation modes.