Arpanoid by EarthQuaker Devices
The Arpanoid by EarthQuaker Devices is an a polyphonic pitch arpeggiator. In simple terms, you play one note, and this pedal plays a scale based on it. Choose the scale type (major or minor), direction (up, down, up and down, and random), number of steps, and tempo. Requires 9v battery or AC power adaptor (not included).
The Arpanoid retails for around $225 and is available from these vendors: