Metacosm is a framework for computer role playing games in Java. An example called ZHeroeZ is also provided. Some of the project's objectives are object oriented design, world persistency (all actions should have a long term impact on the world), coherent time management (time is the same for everybody in the same world), a graphical editor, and choice in client user interface. All creatures can be player or AI controlled by design.
KMuddy is a MUD client powered by KDE. It aims to be the most powerful, fast, feature-rich and easy to use MUD client. The current version already offers many features such as powerful external scripting (which supports very many languages such as Python, Perl, Lua, and C/C++) and its own internal scripting. It has split-screen scrolling, command history, an auto-mapper, buttons, gauges, logging (both in plain text and HTML), multiple connections, regex support (on both aliases and triggers), and much more.
NFMCA is a MUD client with basic features in the form of a Java applet. The GUI is written with Swing, so one needs Java Runtime Environment to run it. Besides running in a browser as an applet, it also supports standalone mode. Features include dealing with a limited set of ANSI codes (foreground colors and underline), multiple languages (as you could expect from a Swing program) and font changing (two features which are absent in some much more complicated clients), smart scrolling, command history, detachment ability for applet mode, handy selection/copying mode, and more. It supports English as the default, and Russian for Russian-localized operating systems. NFMCA should run on any platform (tested on Windows and GNU/Linux).
Obsidian is an extensible virtual world system with a fullblown multiplayer client-server architecture. It is not a VRML2 world system, though there are plans to include VRML2 as a world data format. Note that active development on this system stopped a while ago, though the code is still available for the curious. The code may not even compile on current Linux or Irix systems.