Velxio: Open Source Embedded Emulator
Velxio is interesting because it is not just an Arduino sketch runner in a browser. It tries to put the firmware, the circuit, and the instruments in the same feedback loop: AVR, RP2040, ESP32, RISC-V, and Raspberry Pi targets on one side; SPICE-style analog simulation, GPIO, ADC, serial output, oscilloscopes, and custom chips on the other.
That makes it a useful link for the systems/tooling pile. Good embedded work depends on seeing where the abstraction crosses the hardware boundary. A simulator like this gives learners, reviewers, and coding agents a tighter place to test whether the software and the circuit agree before real boards, probes, and smoke enter the room.