Instructables has a great entry on a homemade 3D Chocolate Printer, made from LEGOs (with some custom work). Its very rough at the moment, and the maximum geometry is limited by the fact that there isn't yet a way to work in support structures, but it's a great start. There are some movies on the site of the device working, as well as step-by-step pictures of its construction (in the Instructables way).