This is the smallest Arduino-based design so far.
Make your own 1-x1- 22 I-O pin Ardunio Compatible
When you think about it (provided you know what Arduino is about) the micro controller takes care of most things in the design, and power supply, USB etc can be taken out. Consequently, there’s not much more than a surface-mounted micro controller on the PCB.
For obvious reasons it’s not pin-compatible with any existing Arduino board, but this could e.g. be used in small battery-powered control/sensor solutions with a tiny custom board.
Yet a disclaimer: “This looks hard, can I just buy one? You should just buy an Arduino Pro-Mini. Unless you want to buy about 500, it will be cheaper. There are some differences see the last step for a comparison.”