PhotoFlip! (free) is a really clever idea that lets you add notes to any photo. By just creating a shell around your existing photos with metadata, it doesn't re-copy the photos wasting space on your iOS device. Notes can be at the top or bottom of the photo, and photos can be shared with or without notes through the app via text messaging or email. The notes can be synched to the identical photos on your other iOS devices through the cloud.
The downsides are the app doesn't recognize anything in your photo stream, and when you view your photos through the app you see everything, whether it has notes or not. Photos with notes have a subtle page curl at the bottom right, but it would be better if the app only showed you photos you've tagged with notes rather than everything.
Still, I've found a lot of use for PhotoFlip!, and it is certainly easy to use. No instructions are needed. Everything is obvious on screen. One icon lets you move the note to the top of the photo instead of the bottom. Another is the standard sharing icon that lets you send your photo to others.
PhotoFlip! has the kernel of a really good idea that just needs some extending to make it even more useful. The app is universal, doesn't require setting up an account and just works. If you can live with the limitations you will find it useful. If the organization of tagged photos is tweaked, and if photo stream gets supported the app will be a complete winner. I'd also love to see it sync with iPhoto, but that's not a trivial update.
PhotoFlip requires iOS 7 or later, and it is optimized for the iPhone 5.