Fun! I've been working on a similar free software project to unfold generic STL files for laser cutting: https://github.com/osresearch/papercraft

The problem is that most models on thingiverse produce designs that are very, very difficult to fold up. Even low-poly things (like the Stanford Bunny) are almost impossible.

Not open source, but a popular piece of software for converting 3d models to paper:

http://www.tamasoft.co.jp/pepakura-en/

Cool! I will try to draw something with this builder[0]

[0] http://voxelbuilder.com/

Awesome! Do you have a picture of the fully-built Mario?

This is basically magic to me. Really cool.

Really cool!