Gene Hacker created a 3D printer out of LEGO. It’s like a lo-rez reprap that operates in polar coordinates instead of the more familiar cartesian system. This is still very alpha and his demos show some promising 3D blobs of frosting. With that in mind, however, most of the basic functionality of a rapid fabrication system is here. Hacker posted the necessaries to Instructables, so you should be able to replicate his design and put your mind to making it even better.