I used to MUD on realms of the dragon a long time ago. I was an immortal on one (a coder, pretty much). Helped me learn a little C. I will never go into an MMO though, I got way to wrapped up into that crap in college.
I used to play the AvP MUD and Redwall Muck, but gave them up as I simply didn't have enough time anymore, plus I'm a casual gamer so I can't compete against the hardcore 24/7 gamers out there (which is why I dont bother with WoW or anything like that). In the case of Redwall, I simply outgrew it. When i was younger I also was on FurryMUCK, but after a while all the yiff, etc and the people there began to freak me out (i mean, some of them BELIEVED they were animals in human bodies... )
that's kinda creepy (The believing crap) but yeah i was damn addicted for a while....then one day i just stopped because i didnt have time. but it was so much fun! staring a bits of text flying by...lol ohh gosh what was that other one? it's website based and you build likea country, and you can be a human or whatever, and you attack neighbouring countries and you form alliances etc, and you can buy dragons? argh what's it called??? like you produce citizens and you train them to be thieves, warriors, mages, whatever.
World of Warcraft for me I use to play a mud called Legend of the Red Dragon (L.O.R.D.). It was one of the original muds that's been around since the 80's. Aside from that, I'm a big fan of the Fallout games, particular Fallout: Tactics.
I have played quite a few different MPOG's Which are text based. I was addicted to it at one point. And still play a few from time to time. They are such a waste of time tho!
it gets worse when you start getting austracised (sp) by ppl because you happen to be sane... sounds like Swirve's Utopia... http://games.swirve.com/utopia