Microduino actually isn't the smallest Arduino clone on the market. The Femtoduino, boasting the "same power and pin count of an Arduino Uno," measures just 0.81" × 0.6".

The Arduino Uno-compatible board has an MCS-51 (often called 8051 instead) instead of the usual ATmega328P/ATmega168. Specifically, [ElectroBoy] uses the AT89S52.