This project an automatic instrument tuner from 1988 shows an innovative use of fairly simple circuit blocks to come up with a useful device for tuning an instrument.
It uses a microcontroller and motor drive to control the string being tuned while listening to the string using a microphone.
I have never actually seen one of these and it sounds like a great idea for a project also the project uses minimal circuity so should be easy to duplicate using modern microcontrollers.