The guy that owns Minecraft was pulling in a quarter million a day as of a couple of weeks back.
$12 or $13. Twenty second download.
Tons of fun. It hits the creativity, exploration, and zombie targets head on. The graphics are old school but I wouldn't have it any other way.
Watch this to start...
http://www.youtube.com/watch?v=ylVtj-1CcggGood series. I watched the first two before I went off on my own. I also use the wiki now and again.
To get to your save games go to Start, Run, %appdata% and you can find them in the Minecraft folder. That way you can back up your game if you want to take chances or fool around. Also you can send somebody else a saved game and they can explore your world and see what you create. We swap them in the clan. It is a blast seeing what others have designed and built.
If anyone gets it, I will Dropbox you a link to my most recent save so you can see my current world.