My favorite is PictureShow, it lets you edit the most of all your pictures, and you can even save the exact way you edited your picture- like a recipe card- for future photographs.
Instagram: I have talked about this one before, best FREE app for the iphone, and you can chose so many different effects!
Hipstamatic: I'm going to have to say this one is my least favourite on of all, it won't let you chose the effect you want it just randomly pics on for you, and the options you get to edit the picture I find is very limited, maybe it's just not as user friendly as I would want it to be- it's still pretty cool though.
Camera Bag: This one was a bit better to use, honestly I think Instagram has a lot more options, but there are some styles that are really cool that Instagram doesn't have.
What apps do you use?