An app that I play with from time-to-time is Perfect Photo. It has a nice group of filters and enhancement tools. Almost all of the tools have sliders to fine tune, which is much appreciated over lesser apps that just apply and go. The undo/redo is handy too so you can play a lot with images to get just the look your going for.
The app is from a company that does a lot of iPhone Photo apps so be forewarned that even if you had the for-pay Perfect Photo app, your going to get a lot of ads pushing you to the pro version of the app that is a touch more expensive (no discount for already paying for another one of their apps).