TinyMARE (Multi-user Adventure Roleplaying Epic) is a text-based MUD server completely rewritten for efficiency using TinyMUSH and TinyMUSE as a basis for its game engine. The server is designed for creating an extensive, novelistic role-playing atmosphere supporting real-time combat, day & night, seasons, and global weather. Using a telnet client, players can log on to explore a virtual world, find magical items, learn skills and techniques, and group together to surpass obstacles you create in your own epic adventure.
CircleMUD is a multi-user dungeon game system (MUD). It is a derivative of DikuMUD Gamma 0.0. A MUD (Multiple User Dimension or Multiple User Dungeon) is a computer program which allows Internet users from around the world to connect to it, create fictional characters for themselves, and then interact with other real users as well as computer-generated entities in a virtual environment.
Obsidian is an extensible virtual world system with a fullblown multiplayer client-server architecture. It is not a VRML2 world system, though there are plans to include VRML2 as a world data format. Note that active development on this system stopped a while ago, though the code is still available for the curious. The code may not even compile on current Linux or Irix systems.
mcl is a MUD client running in a Virtual Console under Linux or using standard TTY routines under any other UNIX. Embedded Perl or Python offer very high tweakability. Other features include huge scrollback, a stable input line with the MUD prompt, Perl regexps for actions and substitutions.