Sonic Vox

Sonic Vox is a cool little 99 cent app that lets you modify your voice. While the interface works well, there's little feedback on what you are doing. And unfortunately there are a few hardware limitations that keeps this app from being perfect.

You use your finger on the screen and move up and down to change the echo and left/right to change the pitch. The sounds that come out are very cool. While it works with just the iPhone, I think the app works best by connecting it to speakers and using the iPhone like a microphone. Here's an example movie from the developer that shows you how it looks and lets you hear some of the sounds.

Great app, and really fun with some speakers connected!