Femtoduino: an ultrasmall 20.7×15.2 mm libre Arduino compatible board | Varesano.net.
By using the QFN32 version of the ATMEGA 328p, 0.05″ connectors, 0402 components and removing everything not strictly necessary, I’ve been able to design and hand build an Arduino compatible board which is very small (20.7×15.2 mm) and ultra light (2g) but has exactly the same computing power of the Arduino Duemilanove or UNO.