The lovely app “Spectral Eye” is not only a neat music-visualizer, but makes jamming along with music not only possible, but really fun. The app consists of a “harmonics-ring”, where the frequencies of the current input sound is mapped to notes (from C, C#, D etc. to B). You can at a glance and on the fly see the main base harmonics of the current track playing. If you start playing the main notes, you instantly “got” the music harmonies. And you can see the play of the overtones as well.
Of course, there are apps in the market, that automatically match the harmonics and apply “automatic to MIDI magic” for example. But the human factor like “seeing the notes and start playing” is sort of charming and makes this a recommended music app!