Trapcode AirPaint for iPhone
This app allows you to paint in air with your iPhone! It uses the built-in accelerometer to estimate how you move the iPhone when painting a stroke in air and the stroke is then displayed on screen.
How to use video on YouTube
How to use video for iPhone
Buy App (iTunes App Store, or search for "AirPaint")
Accelerometer data alone is not enough information to recreate the stroke, so this application uses physics-based methods to guess how your hand moved based on the acceleration data. As a result, it is fairly easy to paint simple shapes like squares and circles, but more advanced shapes are harder and provides a fun challenge.
you paint something nice you can use the iPhone's screen shot capability
(press "home" and "awake/sleep" at
the same time) to save your art piece with your photos.