CoffeeMud is a text-based virtual reality game engine (a MUD). It is a mature, full-featured Java codebase. It includes web (HTTP) and email (SMTP) servers, chat (IMC2 and I3) and web clients, and supports ANSI, MXP, and MSP.
MindGate is a virtual reality multiuser client - server system written in Java. The purpose of the system is to provide communication, persistence, 3d streaming support for users which are imersed into virtual reality worlds - stored anywhere on the Web.
A graphical isometric multiplayer virtual reality server in MUD-style. Rooms and server code can be changed online. At the moment, there is also an IRC link. Clients are available in Java and Python/C.