Add Text to Your Photos
You'll either annoy or delight those who follow you on Instagram/etc. by putting artsy typography over top of your shots of coffee cups, skies with clouds and fields of snow with Over ($1.99 in the App Store).
You can check out their Instagram or Tumblr feeds to get an idea of what's possible.
Sample of Over permalink
Could be neat if used properly. Very annoying if used a lot. With great power comes great responsibility.