Secret Base Design has released Spectral Eye, an iOS App to perform an FFT on incoming audio. The spectrum is wrapped in a spiral, so that pitches that are an octave apart are lined up as rays radiating from the center of the display. This makes it easier to see the notes present, as well as harmonic overtones.
The app is free, and does not have ads. It was a tool used in the development of a few other apps and, as it seemed like a handy utility, Secret Base Design cleaned it up a bit and made it available to anyone who might want to play with it.