Simulate your IO with this Arduino Simulator
ESP8266/ESP32Arduino custom modem<->internet firmware
Open-source RTOS and Embedded Systems Toolkit for C++ and C
RetroPC-SIM is an Arduino project to help to understand how pc works
SDK for connecting to AWS IoT from an Arduino Yún
To be a practical Arduino simulator
ESP32 Online simulator for windows, Mac and Linux - Free from Wokwi