MUD2, the next incarnation of the MUD by Bartle and Trubshaw mentioned in the article is still running and free to play (I believe), http://mud2.com/. I played it on and off from 1994 to 1999 or so. If your character was killed in combat you died dead dead and had to start all over again which made it feel very high stakes when you could spend months building a character only to have to start again.

I decided the only way I'd get it out of my system was to make Wizard which I did eventually (that was quite the phone bill) and I stopped playing a little while later. I've never really got into another online game since.

I seem to remember some gaming service in the UK trying to make a client for MUD2 with some graphics to try and give it more mainstream appeal. It didn't really work. I do remember the conversation of "I wonder if it's possible to make a MUD but with graphics?" came up in the teamroom chatter from time to time.

Now my kids play Roblox, which is also kind of amazing in its own very different way. It has the social element of a MUD (although my kids mostly know the people they play with in real life first), and its a gateway to programming, but all the experiences are far more lightweight and short lasting whereas I think the land of MUD2 has left some kind of lasting impression with me.