I've never gotten into MUDs - too much magic and leveling up. For any science fiction fans who think the world of text adventures isn't your cup of tea, you should try Fazuul. Fazuul is a multiplayer online game. You are stranded on the planet, left to find your way off. There are tools to work with, games to play, and many strange and unusual things.
Here's the start of the game:
"You're standing on the tarmac of a large spaceport, under the glare of a bluish-white sun. To the west you see a large gate leading into an exotic alien city, with glittering towers stretching up into the sky..."