There are already many USB-to-TLL debug boards on the market, but Olimex USB-SERIAL-L open-source hardware USB-to-serial board is more advanced than most with not only Tx/Rx pins, but also CTS/RTS ...
SoundSlide is an open-source hardware USB-C adapter that adds a capacitive touch interface to your laptop or keyboard PC in order to control the volume without having to reach out to the volume keys ...