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.
antioch is a Web application for building scalable, interactive virtual worlds. Begun as a MOO-like system for building virtual worlds, the goal was to take the LambdaMOO approach to creating online worlds, and update it in hopes of attracting new players to an old idea. Like many MOO clones before it, antioch uses Python as its internal scripting language. This provides a powerful environment for game authors, while a flexible object model allows for creation of complex in-game objects.
AIME is a C++ MUD engine with an OO design, designed to provide utmost flexibility for the designer to create complex worlds. It has a fully-functional builder port, online interpreted specials code, and a flexible levels/quest/profession track system to allow the game owners to set up any world they can imagine.