Filterstorm This app is like Aperture for the iPhone and lends a great many set of tools for perfecting your images. In addition it has some effects lenses, but more in the photoshop sense and less in the hipstamatic mode.
Camera Flash Deluxe This app helps you instantly brighten up your photos. Great for adding some "pop" to you dimly lit photos.
Trimensional Though this really belongs in a "freaky photography" list, I found this too cool to not include. Get your subject in a dark room and fire this baby up and you'll have a 3D 180 degree rotating image of the subject. Freaky fun!