A free program for Android, by Willey Piano.

PianoMeter is the perfect tool for anyone who wants to learn how to tune their own piano.

This app not only teaches you how to tune your piano, but also helps you get to know all the important aspects of piano tuning, such as what exactly you need to do, how to tune to frequency standards other than A=440, and more.

All you need to do is open this app and follow the given instructions.

With this app, you can also tune your instrument to historical or custom temperaments, as well as calibrate the device to an external frequency source.