The term MUD originally referred to a particular game, not an entire
class of them. MUD (Multi-User Dungeon) was written by Richard Bartle
while he attended the University of Essex in Britain. He wrote it in
BCPL on the University's DEC KL10. The point of the game was to gain
points until you achieved the rank of wizard, at which point you
became immortal and gained certain powers over mortals. Points were
scored by killing things or dropping treasure into a swamp. The game
gained some popularity in Britain when a guest account was set up that
allowed users on JANET (the British academic network) to play during
the small hours of the morning each day. Bartle eventually left Essex,
taking MUD with him.  Bartle has since gone commercial. (There are
several commercial MUDs available in Britain, some doing rather
nicely. One is Bartle's MUD-2. MUD-1 is also commercially available in
the US on CompuServe. They call it British Legends.) People at Essex
implemented a new world using the old MUD software called MIST. Later
another game, LAND, was added. MIST is still up today; LAND is now
called TREK. The Essex KL10 is due to be taken out of service this
August, so the original MUD will soon be taking its final bows.

One player of the Essex MUD was Alan Cox, also known as Anarchy. He
wrote (with a bit of help) AberMUD, named after the University of
Aberystwyth, Wales, which he attended at the time. It was originally
implemented on a Honeywell mainframe running GCOS but was soon ported
to UNIX. Its poor design and implementation (all game information was
stored in a shared file, which meant that several processes were
constantly accessing the disc) did not endear it to many system
administrators. Nevertheless, it was the first MUD to gain widespread
popularity. After the source code reached the United States, several
people made enhancements and additions, notably Rich $alz. It now
seems to have found a home at St. Olaf University, where a few
dedicated hackers are keeping it alive despite its general grunginess.
(Don't take this personally, Anarchy - we luv ya anyway. Wait'll I get
to YAMA.)

TinyMUD was originally prompted by a discussion on IRC (Internet Relay
Chat). James Aspnes wrote it and created a monster. TinyMUD isn't a
MUD in the classical sense of the term; it isn't a game. In TinyMUD
all people can really do is create things and interact with others.
(MUD purists -- like myself -- can keep going on this point for
hours. I personally think it's just a glorified chat system.) It has
built up a considerable following, and today is perhaps the most
popular MUD on the Internet. Aspnes' TinyMUD went down when he finally
got fed up with it.

TinyMUCK and TinyMOO are derivatives of TinyMUD, as should be obvious
from their names. Their main difference from TinyMUD is that they are
programmable. I can't be too specific on these, because I haven't
payed much attention. (I don't like TinyMUD, remember?)

LPMUD by Lars Pensj| of Sweden was the first MUD with a built-in
extension language, in this case a subset of C. Wizards in LPMUD can
create new rooms, objects, monsters and commands, driven by programs
of arbitrary complexity. This introduced a considerable level of depth
into the choices open to wizards, and brought some new problems too.
The original LPMUD is still up (host milou.cd.chalmers.se, port 2000).

Ubermud is another programmable MUD. I've not played it so I can't
offer specific comments. The author seems to have mostly lost interest
now that the software is finished. Today, the code is used more as an
example of what can be done with MUDs than an actual production
system. (There are no public Ubermuds running today that I know of.)

YAMA (Yet Another Multiuser Adventure) is the latest project of Alan
Cox (you remember him. Anarchy.) It is actually a program for writing
MUDs. It is extensible, though its programmability is rather lacking.
Of all the MUDs I've seen, it's most similar to the original MUD.
That's enough reason for me to like it. Alan still considers it to be
beta testing and is not ready to release the code, so don't even ask.
(Oh, by the way, this one doesn't trash machines like AberMUD did. In
fact, the client used is telnet.)

And finally, the ever-popular VaporMUDs. There are at least two MUDs
that, according to their authors, are currently being written and will
be the greatest hoopiest most whiz-bang games to ever hit the earth.
I'll welcome these games when they arrive, but for now they're just so
much hoopla and hype. Regardless, MUD development goes on.