Michael created the Hack-a-Sketch -
An Arduino board reads the inputs from two potentiometers (the knobs), and sends the information via USB to a Processing sketch which displays the path of the stylus on the screen. This was extremely easy to build because the Arduino is just running the StandardFirmata firmware. No custom code on the board. The Processing sketch was surprisingly easy to write. Using this really did feel like using an Etch-a-Sketch.
The shake sensing comes courtesy of a mercury switch behind the monitor. Of course a lot of laptops come with built-in motion sensors nowadays – might be fun to tap use those as erase triggers instead.