http://terraria.org/
I spent 3-4 hours (I think,) last night building my own underground fortress. I've built a few surface structures to try and get more people to move into my town. (there are different things you can do to attract different types of tenants, like merchants, arms dealers, etc.) If they move in, you get new and better items. So far, I've failed in my attempts. I have managed to fortify it enough to keep the zombies and demon-eyes out at night though.

I've started mining deeper, and have hit some really interesting areas. It's slow going by myself though, because you have to make sure you can get back out again, make sure the tunnels are lit, etc. or it gets dangerous really fast down there.
MANY PLAYER MFing' COOP!!!!! So when are my friends going to join me at #6 Bishop's Mews? (that's what I called my area) I'm running out of trees up-top though, so it may be a good idea to start a #7 Bishop's Mews level with "Large" map (as mine is only medium.)
Who can play soon?
I'm actually thinking about running a 24/7 server! Password protected, that we can all jump in and out of at will!!!
Over!