"PennMUSH is a freely available MUD-style server that provides a form of textual virtual reality. It maintains a persistent world database containing players, objects, rooms, exits, and programs. People connect to the server and take on characters in the virtual world, interacting with other players from around the (real) world. Common activities include game playing, role-playing, socializing, world-building, etc. These servers have also been used for education, research, and artistic endeavors."

"PennMUSH is a C-based MUD-like server, first released in 1992 and still actively maintained."

"RhostMUSH is a security hardened text-based multiplayer RPG server initially based on TinyMUD. It features granular permission systems, a wide and varied set of tools for building your online world, and a continuously developing codebase."

Kmud is a free graphical MUD client for Linux and other Unix platforms running KDE. It is an useful and easy to use program for all people who want a nice graphical interface instead of a simple telnet for playing MUDs. The goal is to be the best MUD client for Linux/Unix.

Xpertmud is an extensible perl, python and ruby scriptable MUD client. It is based on Qt and KDE. There's a native Linux/KDE version as well as a native Windows/Win32 build. This enables you to use the same script on your Linux box at home and on your Windows box at work...It supports multiple windows (which are scriptable), and you can use triggers, aliases or whatever you want on every bit of text that comes from or goes to the server. There's a stable and intuitive plugin API, so you can extend the client with Qt-based C++ code. A sample plugin for the btech MUX is included.