Do you remember the app Singing Fingers yet? Slewpi does about the same, but with a built-in synthesizer instead of your own recordings. On the iPad, iPhone or iPod touch you draw on the touch screen, and the synthesizer starts playing. A video says more than words in this case:
Slewpi is available in the App Store and costs $ 0.99 (€ 0.68).
Via: wire to the ear