See this new board that debuted on kickstarter: amazingly small, this product comes to join the ever growing Arduino family.
The Microduino series is a 100% Arduino compatible open source hardware, compatible with Arduino IDE development environment and existing Arduino sketches.
Its size is about a quarter (25.40mm X 27.94mm / 1.0inch X 1.1inch), much smaller than the original Arduino board.
All Microduino boards adopt a uniform U-shape 27-pin standard pin-out (UPin-27). Thus Microduino series can be easily stacked together through the UPin-27, and all modules are delivered ready to stack.