Polyplayground by MIGAMO (Mike Gao Mobile)

This app is great for the producer, composer, music theorist, and student. It makes understanding music theory easier, because the spaces are isomorphic, meaning that chords are the same in all keys. One only needs to learn a chord shape once to use it in any key.

The default 3:4 Balzano space has unique symmetrical properties that make it the ideal way to visualize and play harmony. The only drawback is step-wise motion, which has been addressed in the newest version of Polyplayground with Swipe Mode 3/4 (scalar swipe), as well as graphical cues for adjacent scalar notes.

This app has a sampler and synthesizer engine with separate pitch bend for each finger, but is designed to be used as a MIDI controller. For live performance, the app takes MIDI input, allowing you to visualize what a jam partner is playing. You can also stream chords you have already played into the device so that you are never lost if you want to improvise.

You can press the Horizontal/Vertical buttons to change your pitch space.
Guitar/bass players can use guitar mode, which changes the pitch space to a 5:1 (fourths by chromatic) for their familiar layout, while cello players can use 7:1 (fifths by chromatic).