Amazon.com: Beetle - The Smallest Microcontroller for Arduino : Electronics
Introooducinggg micro45/85! (Unofficially the smallest arduino compatible dev board) it features usb, bread board compatibility, power filtering/ power led! All under 1inch in every dimension. : r/arduino
ATTO - The World's Smallest Arduino Compatible Board - Electronics-Lab.com
PICO, the Smallest Arduino Compatible Dev. Board from MellBell Electronics on Tindie
Beetle - The Smallest Arduino Board With Bluetooth 4.0 - Demo Board - AliExpress