For those who aren't familiar with them, a social MUD is a text-based virtual reality of sorts in which users can get together, chat, design new virtual areas to chat in, program objects to do useful and amusing things, explore and see what other people have created, and so forth. I prefer them over IRC, as they tend to be quite a bit more flexible and fun to play with.
One other thing: somebody would need to provide reliable hosting for something like this to work; residential broadband doesn't cut it.