This project was developed by the Computing Culture Group at the MIT Media Lab.
It's now called #6!
It's a 2"x2" (5cm x 5cm) one-side printed circuit board that supports the very robust Atmega32 microcontroller. Because it's one-sided, you can make them at home with Press and Peel Blue a clothes iron, and Ferric Chloride from Radio Shack. It can be configured minimally with just a tiny handful of components yet connect to so many different things.