One of the great things about the Arduino is that it's totally open source so you can make your own variations on it. Case in point, DIYer Johan Kanflo's AAduino, which comes in at the size of a AA battery. Kanflo's project is a wireless Arduino clone that's meant to operate in a way that doesn't take up a lot of battery juice, so it's pretty low power. Still, it has a lot packed into it and it should be able to power all kinds of fun Arduino projects. This isn't an easy thing to make on your own by any means, but if you head over to Kanflo's GitHub page you'll find the full guide to make one for yourself.