Here are some of the iPhone Apps that I use most:
Slow shutter – I use this when I want to do pans or swipes with the iPhone.
HDR – This has been my primary app for shooting HDR images.
Pic Grunger – I use this for creating down and dirty grunged or aged images.
Hipstamatic – When I want to create retro looking images. A camera with built in post processing effects.
Autostitch – When you want to capture a panoramic.