This morning I followed a link that _why shared with us, a lo-fi guitar pedal built around an Arduiono. Kyle McDonald made said pedal; he also made I Eat Beats, a drum machine gizmo built with a screen, Processing, and skittles. He also published instructions for how to build a 3-D controller. From there I found a coin-slot detector, again built with an Arduino.
Moral of the story: there is a ton of cool stuff going on with Processing and Arduino. Just let the links light your path.