The original project[1] was a very simple 500 line implementation of Minecraft. There is a fork[2] of that project with 8000 lines of code and many more features.
I used [1] above as the basis for a few sessions at our local coderdojo, it gave an opportunity to get the kids to hack some python and get visually interesting results.
http://damianmooney.wordpress.com/2014/03/08/a-maze-in-pytho... is the kind of thing we ended up working on
[1]https://github.com/fogleman/Minecraft
[2]https://github.com/boskee/Minecraft