I love a good iOS photography app, I use quite a few on a regular basis and have experimented with loads. I've recently come across Dubble, a new double exposure app that randomly pairs your image with that of a stranger. Often the outcome is a bit of a mess, but with a little bit of persistance and plenty of luck there are some really nice outcomes. It can be quite addictive once you get in the swing of things.
If you are feeling adventurous, try double dubbling.
The app is easy to use and free to download in the app store.
Check out my profile: dubble.me/bentopliss