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.
ymusk stands for Yenzie's Mush Klient. It is a mush client written in Perl, and it is built around the Net::Telnet and Tk modules. It is meant to be a simple program which is easily customizable. Its features include logging, macro definition, speech filters, command history, and ANSI colors.
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).
Battletech MUX is an Internet based, real-time rendition of the popular Battletech board game. Players take to the front lines in their very own mechs, tanks, and similar implements of destruction. Featuring an internal scripting language called SoftCode, game admins may sculpt their sites into whatever they can imagine, be it small-scale combat or massive, galactic conflict.