This app looks absolutely insane. Check out this video.
Here is the description from Konkreet Labs:
PERFORMER is a music control and performance instrument, made for multitouch iOS-based devices, designed with two aims: one, a controller that utilises multitouch technology to the full; no faders and knobs, instead an intuitive control of software instruments on your computer, via manipulation of abstract objects on your multitouch device. And two, an instrument for electronic music performance that aims to be visually shared with the audience, to create a live show that is indeed alive!
PERFORMER is a controller. It does not generate sound by itself.
It’s $24.99, so you know it has to be pretty awesome right? The only drawback is that you actually have to be connected to an instrument.