ProCam:

ProCam 3 offers unparalleled control and quality with DSLR-like camera functionality and full featured photo / video editing capabilities.

I use ProCam in my beginner photography classes to show students in real time how making adjustments to shutter speed, exposure, white balance, ISO, etc., affects their images, whether they shoot with the iPhone or a DSLR. The app offers various in-app purchases and free tutorials on the web site.