Arduino mini Pro
My first Arduino board, an Arduino Uno, is fine but huge. So I looked for a more compact board: the Arduino Pro mini.
It has the same features and pins except the USB connection is apart. There's enough space into a fischertechnik 9V red box to pack 2 I2C connectors and 3 serial ports.
First drawings confirm it was possible to pack all on a 6 x 3 cm board.
After some soldering, final result is here, either on a red box or a yellow one.