There are two types of iOS photo editors: the type that lets you fiddle and adjust images in very precise increments, and the other type that provides a way to enhance your photos with a minimum of clicks.
Prettify (free with some in-app options) belongs to the latter category, and it does its job with flair and ease. Open a photo, click enhance and you get Hi-Def, Illuminate and Color Fix options. Each enhances the image in a different way: Hi-Def appears to adjust curves and saturation; Illuminate adjusts brightness and contrast; and Color Fix rebalances the color. Each fix can be used separately or in an additive way with another fix.
Beyond those fixes there are many Instagram-like filters. There's also a tool to defocus or sharpen selectively, and there are the standard cropping and saturation tools. Portraits can be retouched with a teeth-whitening filter, red-eye removal and a blemish eliminator.
The app does a nice job at every task and is quite attractive, with beautiful gradient backgrounds and very contemporary icons.
The app is free, and I saw the occasional advertisement. An in-app purchase of US$1.99 removes the ads if they annoy you.There are other in-app purchases for adding text, frames and stickers to your photos, but many iOS photographers will most likely have other options for those effects and can avoid the purchases.
Prettify uses the tried-and-true Aviary photo editor, and then adds some additional tricks of its own. It is an excellent quick-and-dirty photo editor, and it runs Aviary in a really good-looking wrapper. If you are looking for something easy, attractive and free to make a good iOS photo great, Prettify is worth a download.
Prettify requires iOS 7 or later. It's not a universal app, and it is optimized for the iPhone 5.