Minecraft is a 3D sandbox game with a very low poly graphical style. Everything in the world is made out of boxes that you can destroy, harvest and place. The game has creative and survival modes; In Creative mode you have an infinite supply of blocks which allows you to build whatever your heart desires, in Survival mode you have to work for those blocks, crafting tools to make your life easier. Oh, and monsters will be trying to kill you while you're at it, so you'll have to build bases to survive.
A Java Runtime Environment >= 1.6/6