Here is a list of all incarnations of MUD which made it into production; there are others which didn't get that far, but non-disclosure agreements prevent the naming of the guilty parties... The dates are from when the game was actually playable by real players, rather than from when coding or beta-testing started; a few of these may be wrong, so if you know better feel free to request corrections...

Date(s)	Site	Version	Nickname(s)	Notes
1978	Essex University	1A	none	Test version, to try out the shared memory mechanism. Anyone could play, but only a dozen or so people knew about it.
1978-1980	Essex University	2A	none	Full-strength version. This was generally successful, and was played for fun by hacker and non-hacker alike. It was eventually put out to pasture because the software was getting very hard to maintain, and the memory constraints of the system meant it soon reached its maximum quota of rooms/objects/commands.
1980-1988	Essex University	3A	MUD1, Essex MUD
1984-1987	Compunet	3A	Compunet MUD
1985-1991	British Telecom	4B	VAX MUD
1987-present	CompuServe	3B	British Legends, BL
1987	Commodore 64	none	MicroMUD
1991-1995	Wizards' Guild	4E	Dragon MUD2
1992-1996	On-Line	4E	On-line MUD2
1992-1994	NVN (USV)	4E	NVN MUD2
1993-1994	ALMAC	4E	ALMAC MUD2
1993-1997	GEnie, Delphi, CRIS	4E	Kesmai MUD2
1994-1996	MPGN	4E	MPGN MUD2
1994-1996	Interplay (Engage)	4E	IPLAY MUD2
1994-1995	IOL	4E	IOL MUD2
1993-1997	SoNet	4E	SoNet MUD2
1997-present	mud2.com	4E	mud2.com
1997-present	Wireplay	4E	Wireplay MUD2