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.
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.
XMud is an extensible MUD, inspired by LambdaMOO and LPMUD. It allows users to extend the system at run-time by creating new objects. Objects are Java classes that follow certain rules with respect to manipulating the state. It was designed with with scalability in mind; the system may be composed of N servers and M clients. Each server is nothing more that a state manager that controls reading, writing, and locking state elements. Methods are executed at the client level and generate state change requests that are sent to the server.
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).
Tinyfugue is a very popular MUD/MUSH command line client rich in functionality. When using tinyfugue in a graphical environment the only thing it lacks is the peripheral buttons and options that are available to graphical clients. Gtinyfugue aims to embed tinyfugue within a GNOME wrapper that provides the GUI widgets that make tinyfugue easier and more comfortable to use on a day-to-day basis. Embedding tinyfugue rather than using simple telnet gives the gtinyfugue user the advantage of the depth of tinyfugue functionality.