CircuitPython
Adafruit's beginner-friendly fork of MicroPython, where the board shows up as a USB drive and the code reruns every time you save the file.
The fork appeared in 2017 with Scott Shawcroft as lead developer. What separates it from MicroPython is philosophy more than technology. The board mounts as a drive called CIRCUITPY, you edit code.py in any editor, and the program restarts on save — no flashing, no toolchain, nothing to install. The APIs are the same on every board, so an example written for one chip runs on nearly all the others.
The price of simplicity is that much of what makes embedded work hard has simply been removed. Interrupt handlers and threads were left out for years on purpose, as the two most reliable ways for a beginner to shoot themselves in the foot. Anyone who needs them moves to MicroPython or C. Installation is usually a matter of dragging a UF2 file onto the board's bootloader drive.