Darkroom

Darkroom's most impressive feature is the ability to make curves adjustments, similar to Photoshop on the desktop. You can individually adjust the blacks, shadows, midtones, highlights, and whites of any image with a simple up or down slide of your finger. The app also allows you to adjust those characteristics across individual RGB channels; for example, you could increase the amount of blue in the shadows, or decrease the amount of green in the highlights. It's the app's first paid feature (with more to come later this year), and it's essential to the experience. The $2.99 price tag sounds steep, but it works great and is really fun to use.