Smule has AutoRap (iTunes Link) for the iPhone, iPod Touch, and iPad in the iTunes App Store for FREE (regularly $2.99) for a limited time.

YO. This is AutoRap. It turns speech into rap, and corrects bad rapping.

AutoRap maps the syllables of your speech to any beat, creating a unique rap every time. As seen on the Today Show, just TAP TO RAP, and the app does the rest!