Simply Piano helps you learn piano step by step, no previous knowledge required. It works with any piano or keyboard. Simply Piano is meant to teach people with little to no previous experience how to play songs and read music.
Run the app as you play and it provides real-time feedback as you progress through lessons. Just place your iPhone (or iPad) on your piano (or use your MIDI keyboard) and play, your iPhone will immediately recognize what you are playing!
The app slowly progresses from learning a few notes at a time to simple (and later, more complex) songs.
The app doesn’t let you skip ahead between lessons to maximize their effect. But not everyone has to start at square one. If you have some prior experience, you can indicate that the first time you use the app and it adjusts accordingly.
Free: iOS (I had to search for “Joytunes”)