"Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments." - http://www.arduino.cc
Both the code and the designs for Arduino are open source so you can either purchase one from a dealer or make your own. You can find reference designs for different models of the arduino at http://arduino.cc/en/Main/Hardware.