Noise.io is a cool iPhone / iPod Touch synth! Really cheap too. Only 7 dollar.
Features:
Control Surface. Simply choose the preset, then slide your finger over the screen and enjoy the result!
As you move your finger over the Control Surface, the sound parameters are changed smoothly in real time, producing the sweetest noises ever.
Three control axis are available - Up/Down, Left/Right and In/Between (that's the distance between the fingers when a user makes Multi-Touch over the Control Surface).
You can also use the Accelerometer to control various sound parameters.
Noise.io is the first synthesizer to incorporate the ESFM technology - Enhanced Subspace Frequency Modulation.
Noise.io should be in the AppStore on the 25th of August 2008.
Price: $6.99
"Noise.io is the most intriguing application for iPhone/iPod Touch available exclusively at AppStore." Link