My iPhone goes everywhere with me, and I am forever taking photos on it. Its great as a simple point and shoot camera, for photo “reccys” (taken a set of photos in an area you would like to return to with your main camera), street photography, and for recording GPS co-ordinates to apply to your main photos if you can’t afford the correct equipment!
When I first upgraded to the iPhone 4s, I downloaded all manner of apps for photography, mainly all the free ones, but after searching the webby thing, and seeing such great photos posted by others, I now have three main apps which I use all the time.
As a general rule, I try to do all my processing on my phone before uploading to my Flickr Photostream, Facebook etc.
3 must have photography apps for your iPhone
Street Photography using your iPhone
Sussing out a possible photo location