Welcome to the World Wide Web Dungeon
http://www.cling.gu.se/~cl0polau/3wd/3wd.htm
This isn't really a MUD, but is an interesting 3D dungeon you navigate through on the Web. Eventually all MUDs might look like this. The interface provides six arrows; click on the direction you want to go, wait a few seconds, and you're closer to your goal.