The noiseforiphone looks promising as the worlds first official iphone synth, with FM synthesis and gestural parameter adjustment.
The folks over at Noise Addicts have a nice blog entry including video clips and some detailed description of the four-channel FM synthesis model called “Enhanced Subspace Frequency Modulation” (ESFM).
I like that they are purposefully limiting the synth to a monophonic tool instead of a polyphonic mess; it looks like a well thought-out and highly intuitive interface, and for $7 you can’t beat it.
Comes out later in August, should be a fantastic addition to the rig. I used to have an old yamaha pseudo-FM synth, lost it in the move from Chicago to LA. It will be fantastic to have FM-type synthesis at my fingertips again.