PICSimLab is a realtime emulator for PIC, Arduino, STM32, ESP32, ...
Real Time Electronic Circuit Simulator.
AVR simulator IDE
Open-source RTOS and Embedded Systems Toolkit for C++ and C
A simulator for Atmel's 8/16-bit AVR microcontrollers
rp6simul is a simulator for the RP6 robot (AREXX Engineering)
Create your own cockpit for flying in FlightGear and other simulators.