LaDiDa is a strange little app that came out a while back. It's basically a "reverse karaoke app," in that, instead of playing music that you can sing along with, you sing, and it'll play the music back to you. The technology seems based on the same idea as Microsoft's MySong -- you hit record on the app and then sing whatever melody you want (or rap, or just play a tune on another musical instrument), and then the app quickly builds up the rest of the band around you.
It works surprisingly well. I'll spare you the trauma of hearing me sing, but even when I just threw in a few nonsensical bars of an improv song, the app did an admirable job of backing up my tune with drums and chords. If you do hit on a worthwhile tune, you can save it on your iPhone, and there's even a "Discover" section in the app where you can hear what other users are recording and rating.
The app allows you to create a full song without any musical talent at all. While it doesn't guarantee that you'll create anything worth listening to, if you've ever wanted to write a song but don't know your sharp from your flat, LaDiDa can help. The app is $2.99 for the iPhone in the App Store.