Almost every owner of an iPhone is taking pictures all the time, but most of us won't take the time to improve them after the fact. Apple provides some rudimentary tools for editing, and there are tools ranging from quick and easy to almost Photoshop-like in their power and complexity.
I'll put Perfectly Clear (US$2.99) in the quick and easy department, and add effective as well. You can take a photo from within the app, or open an image you have already taken. The app automatically applies some fixes, and gives you a sliding bar so you can see the before and after. Some additional features like "fix dark" and "fix tint" can be added, and there is a "beautify" button for faces. If you want to do even more, there's even a "tweak" control, that brings up additional fixes for things like sharpening, noise removal, eye enhance, teeth whiten and skin smoothing. There are complete explanations of the filters on the developer website.
I tried the app on a variety of photos, and in general the app improved upon the original unretouched images. Usually I added a few more enhancements, which takes away a bit from the "one touch" simplicity, but I appreciated the ability to make more changes. The app seems very careful to not overdo the editing. I couldn't drive exposure into clipping, and the sharpening controls were subtle without adding noise to the image. There was even a "de-purple" filter that removes the purple lens flare that occurs if you take a photo with the sun just off to one side.
One thing missing from the app is the ability to undo the last change. You can go back and start again, but if you are doing multiple enhancements, you can't just take the last one off. That seems an odd thing to leave out given the overall power of the app. Perfectly Clear is a good app, and it gets good reviews from users. With a useful undo command, Pefectly Clear would be perfect.
Perfectly Clear is a universal app, and it requires iOS 7 or later. It's optimized for the iPhone 5.