I got a little bored with my world, I'm waiting now for Notch to fix the biomes so that snow and ice return in the snow biome. Right now I want to build a giant igloo base in the large tundra area to my south, but if I dig the snow up it becomes a regular area.
So I decided to give multiplayer a try, and I found a good server at game.minerealm.com
that suits me fine. It's open to all (no registering) it's large, the mods come down hard on griefing, and you're not allowed to use hacks, you have to gather all your resources. I spent the last day building a replica of my house in singleplayer for some reason.
It's pretty empty at the moment because my singleplayer house is filled with chests, I have yet to find papyrus to make some bookcases, there's no sheep on multiplayer to make paintings or a bed, and fire is disabled so my fireplaces are pointless... but it's okay. Some of the other stuff on that server is amazing, I was especially impressed with the blinking tower as I walked past it at night.