STM8S chips are probably the cheapest microcontrollers that you can get. A single chip from ~20¢, or on a board with pins, a button and LED ~60¢. Can’t really beat that.
There’s an OK toolchain for development on Linux consisting of the small devices c-compiler (sdcc) and stm8flash, a free driver to work with the proprietary stlink devices which you needed until now to be able to flash STM8S MCUs.
Enter esp-stlink: a free and open-source implementation of the SWIM protocol that’s used by stlink devices to interact with the microcontrollers. It runs on the popular ESP8266 microcontrollers. The software comes with an updated version of stm8flash, so it’s very easy to integrate into your existing workflow.
esp-stlink thus fills the missing link for an open STM8 development toolchain.