Meet the Danger Shield. It was created by Zach Smith and demoed first at NYC Resistor.
What is it for? Whatever you want to do with it!
No, but seriously, it's a shield (meaning it fits on top of the Arduino, connecting to every pin) and provides a multitude of input and output options for writing code against. The biggest portion are the three slide potentiometers with built in LEDs in them. They are handy for relaying analog data back to the Arduino which can then be sent to the computer for further processing (say, Ableton live for performing music live or to a MIDI device).
See Zach tell you for himself.