We had an introductory session on programming Minecraft on the Raspberry Pi.  This edition is completely free and is now included (I think) by default in the Raspberry Pi Raspian distro.

There is a simple python API which lets you manipulate the game world: add blocks, remove blocks & move Steve (the main character).  For more details see: http://www.raspberrypi.org/learning/getting-started-with-minecraft-pi/.

Minecraft Raspberry Pi