The iPhone offers so many great photography tools. Two of my favorite apps are Tiffen's Photo fx ($3) and Aestesis's Lo-Mob ($2).

Original iPhone image after polarization processing in Photo fx:

Then applied with some of lo-mob's retro and experimentation filters:

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iTunes links:

Photo fx:
http://itunes.apple.com/us/app/photo-fx/id300630942?mt=8

Lo-Mob:
http://itunes.apple.com/us/app/lo-mob/id334581568?mt=8