Lomo Lomo Pro
This app is very much like Hipstamatic and Instagram, but better in that not only does it give you the great retro analog effects and the social networking uploads, but more importantly, photos are saved in full resolution on the phone for possible later printing. That seals the deal for me.
Some cons are that you have to buy different cameras for various effects ($0.99 each), and that it feels like you are shooting on a plastic camera (in that you are looking through a tiny view finder). That last con is actually a plus for me. Worth checking out: LOMOLOMO PRO