Pro Mini - ATMEGA 328P - 5V 16Mhz - Compatible Board
AVR basics: reading and writing GPIO pins - Machina Speculatrix
Amazon.com: OSOYOO Pro Micro ATmega32U4 5V/16MHz Module Board with 2 Row pin Header Replace with ATmega328 Pro Mini for Arduino : Electronics
Amazon.com: ALMOCN 2pcs for Nano V3.0 Module ATmega328P 5V 16MHz CH340G Chip Microcontroller Development Board USB Cable for Arduino : Electronics
Amazon.com: ATMEGA32U4 5V/16MHZ Module with The Bootloader for Arduino Mini USB/Micro USB with 2 Row Pin Header for Arduino (Micro USB 3-6V) : Electronics
Amazon.com: ELEGOO Nano Board CH 340/ATmega+328P Without USB Cable, Compatible with Arduino Nano V3.0 (Nano x 3 Without Cable) : Electronics
Attiny85, Attiny84 and Atmega328 Programming Boards for Use With SparkFun AVR Programmer. : 6 Steps (with Pictures) - Instructables
Arduino Mega 2560 Pro Atmega CH340 - PCF Electronics