Arduino is an open-source physical computing platform based on a simple i/o board, and a development environment for writing Arduino software.
Arduino can be used to develop interactive objects, taking inputs from a variety of switches or sensors, and controlling a variety of lights, motors, and other outputs. Arduino projects can be stand-alone, or they can be communicate with software running on your computer (e.g. Flash, Processing, MaxMSP.)
For more information, visit Arduino.cc