Alessandro Lambardi of 5volt built this excellent MIDI Piano Instructor to help his son play piano. Designed as a visual aide for learning to play new songs, it shows which keys to press by lighting an LED located over the key. He used an Atmel processor to decode an incoming MIDI signal (from either a computer training program or a remote instructor) and light the appropriate LEDs. He has an excellent write-up of the project on his site, explaining everything from the concept to how the circuit was built.
I’m quite impressed with the quality of the build, and his resourceful use of materials such as the LED bar made from a cable raceway.