PhotoAppLink is an open source library allowing your iOS photo app to launch other participating photo apps while passing along the current image.
This will allow users to easily process an image using a combination of multiple photo apps without having to save intermediate images to the camera roll, quitting one app, launching the next, loading up the intermediate image.... Instead the user simply selects a "Send to app" option which presents all compatible apps in a nice interface. The selected app is then launched and the current image is passed along so that the user can keep editing it in that app immediately. This provides a more fluid user interface to the user and encourages the use of compatible apps.