Hipstamatic Hipstamatic is just what it says, the hipster's camera. It looks like you're using a Diana manual camera, except you're using your phone. Much easier.
IncrediBooth IncrediBooth is made by the same people that make Hipstamatic. It only works with iPhone 4 and they really really do look like photobooth pictures. Being obsessed with photobooths, this works for me.
CameraBag This is a great app for photos you've already taken in your phone, but you want to change after the fact. It also gives a cool old feel to the photos.
Format126 This is super similar to CameraBag, but it has a few more options. In addition to that, you can click the options all on top of the photo and see how it will change the photo without having to scroll through options.
Duplicam for iPad Duplicam is great if you have it on an iphone and ipad. It's like a remote for photography.
Adobe Photoshop Express The PhotoShop app is great not because of the options it gives you to change your photo, but because it has cropping. You can also directly upload to different places from the app.
PhotoCalc This is a great app for any photographer. You can choose the real-life camera that you use in this app and it can give you f-stop information, aperture, subject distance, etc etc.
ShakeItPhoto Turn your photos into Polaroids. Nothing better.