I'm always on the lookout for free photo apps that do a good job, and Gradients for the iPad or iPhone (free, universal) fits the bill. The app lets you add gradients to existing photos with almost infinite variations and control. I often create gradients in Photoshop, especially to tone down the sky in landscapes. It's nice to have a lightweight utility to handle that simple task.
There are 46 filter styles available, and each one is customizable in parameters like opacity and tilt. You can turn photos into black-and-white images. You can even create a custom gradient and save it for future use. Unlike so many photo apps, Gradients doesn't throw in-app purchases at you. Everything is included. When your editing is done, you can save your photo, or share it via Twitter or Facebook.
Editing is easy, and as you make changes on the Gradients control panel you can see a thumbnail of your image. The only thing I found lacking is any kind of undo feature. You can tap on a new filter and the old one will go away, but there is no way to return to your starting point without reloading the image.
Gradients is a universal app and optimized for the iPhone 5. It requires iOS 6 or later.