Its available from the iTunes App store for around $US4.00 and has to be of the nicest little additions I've made to my iPhone in recent times. Hard to put your finger on just why this works, but I do know that everyone who touches it is enthralled (and by this I also mean, those who are not musicians or usually have that little interest in music technology).
The user (performer?) is provided with a colored screen and a quiet drone. Tap the screen in various places and different tones play depending on where the screen is tapped. This loops, creating a unique piece of music on the fly, and one that changes gradually on its own. It's also beautiful, with the tones appearing as colored spots that slowly fade.
An interesting idea for musicians - to make such little apps/pieces of interactive music for a different kind of 'prod-user' audience - quite different to the old record consumption culture? For more on developing iPhone Apps and distribution, see the iPhone DevCenter.