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.