I’m a complete iPhone Photo App Junkie (IPAJ) and have over four hundred (and counting!) of them. Here is the latest cool one I’ve found:
infltr is a camera app that lets you add filter before you capture a picture. Move your finger and see the filter change in real-time. Pan across in any direction to discover infinite filters.
Affiliate link: infltr: Filter Photos with over 5 Million Color Hues
The only downside is the filters only occur on live shots – that is, before you take the photo. You can’t (yet) import photos from your camera roll and use the infltr engine to transform the image.