Minecraft in the classroom: http://minecraftedu.com/

Programming mod used at CornCon: http://www.ComputerCraftEDU/
Computercraftedu getting started guide: http://services.minecraftedu.com/wiki/ComputerCraftEdu

Playing at home:
A personal copy of Minecraft is required: https://minecraft.net/
ComputerCraftEDU mod & tutorial world: http://computercraftedu.com/#getting-started (Click "I'm a Player")
Requires version 1.7.10 of Minecraft Forge : http://files.minecraftforge.net/maven/net/minecraftforge/forge/index_1.7.10.html
Instructions with screenshots for Forge installation: http://www.minecraftmods.com/how-to-install-minecraft-forge/

Similar educational programming to build your own world like Minecraft:
Sign up at AgentCubesOnline http://agentcubesonline.com (should be free registration)