LambdaMOO

From LambdaMOO:

LambdaMOO can be compared to a chat room. The MOO, however, is a text based virtual community filled with thousands of people from all over the world. Furthermore, it's comprised of literally thousands of "rooms", which have been created by the users of LambdaMOO, where you may endlessly navigate (walk around) to your hearts content. You walk north, south, etc., from room to room, investigating and meeting people to interact with.

If you've played old DOS text-based RPGs, and liked them, this is the online equivalent for you. You join the LamdaMOO world using a telnet client, so anyone and everyone can join. LamdaMOO also played host to the infamous Rape in Cyberspace case.