The only two I'm aware of are iVoxel and Improvox (not counting 'I am T-Pain' which actually does work pretty well as an autotune if you ignore all the goofy extra stuff).

I haven't used iVoxel and that looks like the more full featured of the two. I've played around with improvox a bit, but it's just been messing around I'm very much not a singer.

It's very easy to use, looks nice, sounds very good, built-in effects. Not super flexible, though, you choose the key and either "happy" or "sad" which I'm assuming is major/minor and that's it. The harmonies sound pretty good to me, but you only get a choice of a few "styles", you can't set your own chords or progressions. The recording/export features are nice, there's several ways record and transfer your results, you don't need to record it with a separate device.

You'd probably want something a little more in-depth for serious production work, but for just $3 it's worth checking out.