A free program for Android, by keuwlsoft.

This app for Android phones and tablets makes the process of tuning your guitar fast and easy. It’s a single-screen app that starts in auto-tune mode and is very easy to use.

All you have to do is run this app, hit a string, and start tuning your guitar. If you are in the orange region, it’s a good thing. That means you are in tune. Hitting the green region is even better, as it means that your instrument is perfectly tuned.

This app has a pitch pipe function, so it can also tune other stringed instruments.