LambdaMOO

LambdaMOO (and its previous incarnation MOO) was the very first MUD type which could claim to be object oriented. Today, most MUDs can claim this, and though not all use the same basics, the object oriented features are clearly present.

In all, LPmud is the most popular server as far as MUD games go, but when it comes to the more serious applications of MUD servers, MOO has a real advantage. This is mostly a result of the various Multimedia-related projects that have been accomplished using MOO servers.

Papers have been written about these more general uses:

MOOGopher
MOOGopher is a system which combines an information retrieval tool (Gopher) with a text based virtual reality environment (MOO). The main advantage of this system is that it combines the ability to converse with other people with the information retrieval system. The virtual reality aspect of this application tends to be very important, and adds to the user-friendliness of the system. Imagine looking for a document and being able to ask someone...
MediaMOO
MediaMOO is a project of the MIT Media Lab, in which various specific sub-projects are being accomplished. One topic was the programming aspect of MUDs, or MUDs as a context for collaborative learning. It showed that the virtual reality which a MUD offers, can be constructive for the learning process, because one has contact with other people in the virtual world. One can ask questions, show examples, etc...
Social aspects
This paper by Pavel Curtis (also known as Lambda) covers the social phenomena in text-base virtual realities. One of the more important contributions which MUDs can offer to Multimedia is the added aspect of social contact. The notion of "not being all alone" on the net is clearly existant in MUDs, and most other networking applications are specifically lacking this aspect.