Renode
A simulator from Antmicro that runs unmodified firmware on virtual boards, often several at once wired into a network.
Where QEMU mainly emulates processors and a handful of standard machines, Renode focuses on whole systems: microcontrollers with their peripherals, sensors on the I²C bus and radio links between nodes. A platform is described in a text file, and the very ELF you would flash onto the real board loads unchanged. It is written in C# and released under the MIT licence.
The main use is testing without hardware on the desk. Tests are written in Robot Framework and run in CI, the simulation can be paused and stepped deterministically, and GDB attaches as it would to a debug probe. Zephyr uses it to run tests against boards nobody has in the lab. The limit is model fidelity: a peripheral simulated more kindly than the silicon fools you just as effectively as a mock.