Gridditor is an image effects app that smartly uses a gridlike interface to let you swipe through the intensity of each filter, but it's lacking some features found in other apps from this genre.
Like other photo effects apps, you can snap a fresh picture with your iPhone camera or choose one from your photo library. In Gridditor, the photo you use will be the focal point and you can choose four effects (from 22 total) in each cardinal direction. As you swipe farther out in one direction, the filter for that side gets stronger. With this interface, it means you can easily blend effects and know just how much you're getting from each effect.