"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."