I was a big player of MUDs back in the 90s. I probably spent way too many hours staring at green text (when I should have been studying), but I wouldn't trade those hours for anything. Some of my best computing memories of that era are from playing various MUDs, and even 20+ years later I still keep up with some of the friends I met in the games. Many of the MUDs I played on are sadly long gone, but a few are still a…
Can you recommend any for a newcomer to look at?
Some have specialized clients they recommend; I'm a big fan of just using TinyFugue [0] in a shell window, but that's just me. :)
MUD Connector [1] is a great place to start, as it has a few thousand listed with average sizes, themes, reviews, etc. All searchable, so you can find what appeals to you.
As a very general rule, start with the bigger more active ones. The smaller ones can be very fun and you can make great, close friendships with other players, but they also can tend to get kind of insular and hard for a new player to penetrate.
[0] http://tinyfugue.sourceforge.net/ (it's also in most package managers and in homebrew for Mac)