DronePhones is an excellent resource for anyone who practices a musical instrument where intonation is a consideration (e.g. strings, woodwinds, brass, voice, etc.).
DronePhones contains a collection of drones in each of the 12 pitches of the chromatic scale (A = 440 Hz, concert pitch). You can practice scales, arpeggios, patterns and even improvisation in any key using DronePhones as a musical reference!
The idea behind DronePhones is to use the app to play a droning bass note - usually the tonic (1st) or dominant (5th) of the scale, and then play scales and patterns over it. This technique focuses your attention to producing clean harmonic intervals using your ear as a guide, thereby improving intonation.
Of course, as the name DronePhones implies, its best to play the drones through a set of earphones or an external amplifier attached to your mobile device so that you can match the volume of your instrument.