Momoko is a generic, object-oriented, modular, extensible environment for creating multi-user environments. It is suitable for writing MUDs, servers, and community-based Web sites. It is written in pure Java and supports Java and Python as internal scripting languages. Momoko provides facilities for organizing objects into tree-based hierarchies, orthogonal persistence, a transparent distributed objects system, remote access to source code and user files via WebDAV, a telnet-enabled shell for manipulating the object database, and an integrated build process which can be triggered remotely.
MudMagic Mud Client is a GTK/GNOME multi-platform MUD client. It provides aliases, triggers, multiple connections, plugins, MCCP, ZMP, MSP, tabbed sessions, ANSI color, and a game list that can be updated from mudmagic.com. It is configurable on a per-session basis. Plugins include an automapper, database, and note pad.
Multi-User Environment Server (MUES) is a multi-threaded, event-driven Internet game environment server written in Ruby. It is intended to facilitate the building of online multi-player games or simulations, by providing one or more dynamically programmed object environments (worlds), the means to access these environments (using a network client), various useful services or daemons (for creating in-game systems), and an event system (for facilitating the interaction of all the parts).
Myth2 is an implementation of a binary tree and used to run an 'interactive story'. The user traverses down the tree and at each node is given a bit of story and two options to choose which path to follow. If there is no story after the option, the user is given the chance to add to the story. It is written in PHP 4 and uses MySQL for the backend. Output is fully XHTML 1.0 Transitional compatible and fairly flexible.
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.