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

Instructables member Patrick Panikulam has created a versatile wireless Arduino pen plotter and laser engraver, powered by an Arduino UNO microcontroller ...

The Uno Rev3, is a great beginner board as it's included in many starter kits. It's also complemented with a host of online resources that even without the official Projects Book from Arduino, you ...