Not really free
Although Riff Maestro can be freely downloaded from Google Play, you have to pay for the main functions of the app. The all-in-one bundle of $8.99 gives you a metronome that indicates the number of the pace and the ability to adjust the speed and pitch. But perhaps more importantly, after this in-app purchase Riff Maestro has the option to display real-time chords while songs are playing. This works best in pieces with only one guitar is audible.
From the home screen of Riff Maestro, you can press the plus sign to import a mp3 that is on your device. The bottom buttons of the screen let you run certain pieces from a number, so you can learn what chords are played at what rate. You will also find the functions to slow down the number and adjust the pitch. It is self-explanatory and works fine.
On the right is a burger menu that gives access to the settings, your user account (optional), and in-app purchases. Which cost between $4.49 and $8.99 each.
Download Riff Maestro
Via the link below you can download Riff Maestro. The app requires Android 4.1+ or iOS 7.1 and at least 5 MB of free storage space.