PennMUSH for Macintosh is a Macintosh port of PennMUSH, a popular Unix based MUSH server. A MUSH ("multi-user shared hallucination") is a type of mud which is often used for social and role-playing activities. It is derived from "Tinymud", an early mud server. Its distinguishing features are that any player can typically extend the virtual world by building new rooms and objects, and its internal programming language, MUSHcode, which is considered to be fairly easy to learn.
The Macintosh version adds support for unique Macintosh features, such as a graphical User Interface, as well as multitasking control and Setup Assistant to help with configuration.