Programmable circuit board with open-source IDE.
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.arduino.cc
Arduino refers to both the hardware (single boarded micro-controller) and the software (a minimized IDE based on the Processing IDE) which makes hardware prototyping simple and engaging for people with little or no programming experience.
There are also similar pin compatible versions such as: