KMuddy is a MUD client powered by KDE. It aims to be the most powerful, fast, feature-rich and easy to use MUD client. The current version already offers many features such as powerful external scripting (which supports very many languages such as Python, Perl, Lua, and C/C++) and its own internal scripting. It has split-screen scrolling, command history, an auto-mapper, buttons, gauges, logging (both in plain text and HTML), multiple connections, regex support (on both aliases and triggers), and much more.
Reality is a client for Cyanide. Cyanide is intended to be the most dynamic multi-player online role-playing game ever made. This client uses an isometric perspective to present scenes. It is programmed with Python, pygame (SDL), and OpenGL. Full 3D movement is possible, and the server's code can be exchanged online.
ErlyMUD is a rather minimalistic MUD server, written in Erlang and making use of the excellent OTP libraries. The aim is to have solid support for exploration and roleplaying, within a highly fault-tolerant environment where system crashes or reboots are more of an exotic curiosity than a commonplace thing.