Our most recent processing project is to make something interactive, and I thought, what better way to do this than building a Processing version a classic drawing tool, the Etch-A-Sketch. I brought in the ControlP5 library to build my knobs and control my drawing point. Then I added the ability to clear the drawing surface when you rub your clicked mouse over the drawing surface (my implementation of a "shake"). Here's a screenshot:
You can download source code here.