Damn. They didn't include MUME, one of the oldest active MUDs and based on LotR (and a game I've played for twenty years, in fact meeting my wife through that game): http://www.mume.org
Good to see a fellow MUME player in this thread! I was hoping MUME would be mentioned. I'm no longer active but played some from 2002-2008. And apparently I last logged in 28 months ago. Great MUD.
Would also like to give some props to Discworld MUD, which served as both my introduction to MUDs and to the Discworld book series. Still hanging in there, and being actively developed, almost 25 years after it was founded. It sports one of the smartest command parsers I've encountered in any MUD.