Apple iTunes has a Free Photo Adjust Pro iOS App - enhance & retouch dark picture for a limited time.

It's easy-to-use but powerfull photo enhancement app. It can really adjust your dark or bad exposed photos. Unlike many other photo edit apps, Photo Adjust uses local area image processing algorithm. So this smart algorithm can correct a photo even if only part of the photo is dark or overexposed. So it can make HDR-like photos.