May also be called Multi User Collective Kingdom or Multi User Construction Kit, depending on who you ask. The most common is Fuzzball MUCK, an open-source project. Two popular MUCKs are Tapestries and Furry
MUCK. It is a text-based roleplay environment simmilar to a MUD
but without the requirement to kill things and level.
Kevin has thirty-five alts on Tapestries MUCK.