The free version of the fourthcoming full game. Minecraft is a game about placing blocks while running from skeletons. A 3D pixel based world in which you can build and destroy. Also playable in multiplayer and learn more about the full game here: http://minecraft.net/
But the developer reckons he'll have the server back up later today (apparently). If you try the link and it's not working, check back later cause he's on the case!