http://www.minecraft.net/

A game where you can build anything out of an array of elements… anything, rollercoasters, castles, lava moats, etc. It’s like a world building game, but, every night it gets taken over by zombies and you must survive.

Honestly, it’s a hard game to describe, it has elements of Dwarf Fortress, zombie games, super mario, portal, legos… and many of it’s own unique ideas.

The TF2 dev team seems to be enraptured with it:

http://www.teamfortress.com/post.php?id=4130&p=1

And, it is pretty fun. I set up a server, and you can join it here:

http://www.minecraft.net/play.jsp?server=bac3b007f5dd5ab5caf7c20f4c1a5ec9

(or you can find it in the list of servers here, named CoFR Minecraft Server: http://www.minecraft.net/servers.jsp )

It is browser based, so no install needed. If you wish to be admin on the server, send me your registered playername and I will add you (if I know you, that is).
If you purchase the game you can play the alpha versions, which have all the new features, but the server is running classic version, which should be free to play.