Arduino in the size of a AA battery
I like the idea of the form factor as it makes it easy to source a compact case. If it wasn't the case, but rather the compactness, that was particularly useful then it's worth pointing out that smaller Arduino-compatible boards exist. For example...<p><a href="http://www.seeedstudio.com/blog/2013/07/05/smallest-arduino-compatible-board/" rel="nofollow">http://www.seeedstudio.com/blog/2013/07/05/smallest-arduino-...</a>