Trebuchet Tk is a MUCK/MUSH/MOO GUI client written entirely in TCL/Tk. It supports both Unix/X11 and Windows, and includes powerful scripting, triggers, hilites, macros, quickbuttons, keybinds, etc. that you would expect from a good MU* client. It also supports MCP simple-edit, and GUI dialog packages for MOO and FBMUCK6. For Muck users, Trebuchet also supports intelligent colored editing of MUF and MPI programs.
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.
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.
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.
lfgapp is a client and server which provides a looking for group (LFG) service to be used alongside MMORPGs. A player community (player-run guild for example) can run a central LFG server and then group leaders can advertise groups. Players can then look for groups and add themselves to waiting lists for groups.