The Arduino MIDI Library [1]. Back in 2008, I learned C++ to control my guitar effects pedals with custom electronics as part of my engineering degree. It kickstarted my career into tech and open source. Members of the community do all sorts of crazy things [2] with it.

[1] https://github.com/FortySevenEffects/arduino_midi_library

[2] https://diyelectromusic.wordpress.com/2020/09/19/introducing...