MUDs (multi-user domains or dungeons) are basically chat rooms with weapons!(what could be better?). They are also much like text-adventure games and RPGs. These are very fun, and can be addictive.
For a better description of what they are and how to play, I recommend you visit here.

Before you start, you must do one of two things:
1)You can download a special MUD viewer from here, or
2)You can use a special Java MUD viewer available from here!

MUD Sites:
Mud Connector	A MUD search engine with likely the greatest selection of MUDs. Each has a short description. Great site!
Mud: A player's review	A great site with a good selection of reviews for MUDs. Since there are many,many MUDs on the net, reviews are very helpful.
Almost-Complete list of MUSHes	MUSHes are variations on MUDs. This is an alphabetized list of MUDs, which also tells what type, and for some, what famous science-fiction or fantasy series that it is based on.
Book-Themed SF/Fantasy Muds	A list of MUDs that are based on fantasy or science fiction series. If you've ever wanted to be a character in your favorite novel, visit here. They have everything from Stephan King to Terry Goodkind to Tolkien to a MUD based on the wizard of OZ!
Yahoo:MUDs	Yahoo's list of MUDs & MUD related pages that have been registered with them online.
Windows Resources for MUDs	Another place to get MUD viewers for Windows.
Cool Muds	OK place. A list of 3 cool MUDs in the author's view.

Cool MUDs
Random MUD link	This is something that is really cool for when you are bored. It pulls up a random MUD address out of Mud Connector's huge database.
FurryMuck	A popular MUD with a very fun theme: you get to be an animal of your own choosing and type.
ZeeMud	A fun little MUD with a little sense of humor.
Shades Of Evil	I've heard good things about this one, and there are lots of people chatting in here or fighting evil things.