What would you think of a sandbox building game where the maps are entirely procedurally generated, from bedrock up to the trees dotting the map?  What if you could destroy everything in this map down to the furthest boundaries, and build it back up into anything you want using half a dozen tile types?  Oh, and it's a basic 3D engine, written entirely in Java, by one guy.  And he's adding multiplayer.

That's what Minecraft is, more or less.  The entire purpose of the game is to simply have a go being creative at building things using cubes of material which you can texture however you like.  Literally everything in the level can be deleted to make way for your creations, down to the lava flows half a mile underground.  The truly remarkable part is the procedural generation, which creates everything from shallow ponds to huge lakes, mountains and canyons, and even natural caves.  Did I mention it's in Java and it's absolutely free?

Have a go at it at minecraft.net.  Just be careful not to flood yourself out.

Hat tip to Ryx for telling me about this little gem, loath as I am to give him credit for anything except getting everyone killed in L4D by hanging from ledges.