Well, here's a link to a directory of MUDs - over 1700 of them.
http://www.mudconnector.com/
I recommend you click 'Listings' and 'Listings By Category'. There you'll find all the MUDs arranged into groups like 'Manga', 'Newbie Friendly', 'Historical', 'Sci-Fi' etc.
A MUD (Multi-User Domain) is the oldest kind of multiplayer online roleplaying game. It's like a multiplayer text adventure.
They are critisized for not having graphics (though, there are some 'Graphical MUDs' out there, if you look.)
They are praised for letting the players imagination have more free reign.
Much like in a book you can have amazing things without having to worry about special effects budget, so to in MUDs you can describe fantastic places and scenes, without worrying about graphics cards or framerate.
And finally - if you like MUDs, there's always the opportunity to make your own - either in DB or with some of the free engines. In fact, making a MUD in DB would be relatively straightforward