Originally started in Italy as a project to provide students with an affordable, easy-to-use platform for interactive design projects, the Arduino has gained a huge following in only 5 years. It has redefined prototyping tools that were formerly out of reach for the non-engineer, allowing for the combining of art, robotics, automation, and many other themes by the student and professional alike.
The original spirit of Arduino was open source. In the 5 years of it, this is still the theme, which has no doubt contributed to the massive growth and adoption of it.
Recently the documentary for the Arduino and the team who developed it was released, and it really is something worth seeing. (here)