The Wablet app for iPhone and iPad

The Wablet is a physics based synthesiser and noise toy. An animated mesh of springs can be vibrated with the fingers. The Meshs displacement around a path on this mesh is read as an audio waveform, and hence your gestures are turned into sound. Complex evolving drones, chiptunes bleeps or full on noise can be produced, and beautiful undulating patterns created. The physical properties of the mesh can be changed using the sliders, new sound-paths can be drawn onto the Mesh and the App comes with a Max/MSP patch for sending notes over a wireless network and more in depth adjustable parameters.

"It sounds amazing, and the interaction of graphics, touch, and sound is extraordinary" Peter Kirn, Create Digital Music

See it in action:
http://www.youtube.com/watch?v=w_87NkyOmXE