Arduino

Arduino is an open-source physical computing platform based on a simple i/o board and a development environment that implements the Processing/Wiring language.

Arduino can be used to develop stand-alone interactive objects or can be connected to software on your computer (e.g. Flash, Processing, MaxMSP,PD).

The open-source IDE can be downloaded for free (currently for Mac OS X, Windows, and Linux) from
http://www.arduino.cc