There are plenty of great free apps for photo editing. I’ve tried several, including Fast Camera, SmugMug’s Camera, CamWow, Instagram, Pic Collage, Color Effects, and my personal favorite - Camera Plus – which is $1.99 download.
Camera Plus features “scenes” in which you can change the lighting. The effects include color changes, retro and special effects, including an HDR filter. Here is an example of what you can do with Camera Plus. The first photo is the original, untouched, taken with an iPhone.