CamFlip+

CamFlip+ allows you to mirror and invert the live feed from either camera.
When used with a live camera view, you can control exposure, contrast and focus, change between color/mono/3-value/notan, and adjust the white balance, all in real-time.
It also works with saved images, where you can adjust brightness, contrast, view color/mono/3-value/notan and zoom in/out.
You can blur the color and mono live camera and saved images, while the 3-value and notan views can be smoothed to reveal the graphic shapes.
You can then save the resulting images to your camera roll.

This utility is invaluable for artists to see their work with fresh eyes. As an artist myself, I use it all the time.
Any errors in my work jump out immediately. Viewing in mono lets you see values more clearly, while the 3-value and notan views reveal your design.
It even doubles as a basic color meter, to assess the temperature of the light you are working under.
Having this sort of power in your pocket is a real game-changer for artists.