This thread over at the libsecondlife forums describes a Second Life server built by a user, a single sim that can be accessed using the standard client software. As near as I can figure, the server was built by examining the client code (and/or reverse-engineering it) along with the information that is passed between client and server in order to get an idea of what the server code would need to look like. What’s more, an early version of the server code has already been made available as an open-source project.
a tiny tiny step towards a distributed metaverse