tin cans, various hardware, custom electronics
This step by step instructional to build Robot Recipes: Wobble Bot appeared on the Make: blog
"After preparing a tasty pineapple snack, you will have the raw materials to build your own wobbling robot toy. This little guy will shake and vibrate when you turn his head. There is a lot of machining and electronics work in this project, so feel free to tweak the robot recipe if you have better ideas or techniques."