This one conjures up our childhood dream of being Harold with his purple crayon (a dream that still lingers). Designer Billy Keyes' SketchSynth lets you draw your own controls and then use them to control anything that can receive Open Sound Control (OSC) messages. In the video, Keyes controls a synthesizer.
On the project page, Keyes puts SketchSynth into a nutshell like so:
"A standard webcam looks down at the paper and detects both controls and hands. To prevent confusion, there are two modes: EDIT and PLAY. The system basically does nothing in EDIT mode, allowing you to draw controls. When you switch to PLAY mode, the controls are detected. From then on, you can interact with any detected controls."