The best camera is the one you have with you
The iPhone 4 has a pretty good built in camera but the default camera app is a bit limited in functionalities and does not give you much control over your shots. Fortunately alternatives exist on the App Store. The most impressive one to me is Camera+ developed by TapTapTap in association with professional photographer Lisa Bettany. This app is pretty simple to use yet packed with cool features to help you compose and enhance your shots. One of my favorite ones is the ability to set exposure separately from the focus point.
All the photos you take are stored in the 'Lightbox' (Camera+ own camera roll) where you can manage them, display their details (exposure/ISO/shutter speed...) and edit them.. Some of the scene modes and filters inside the edit mode are quite impressive and can really add style to your shots.
Another pretty cool feature: Once your happy with your edit, you can choose to save a copy in your iPhone camera roll while keeping the original in the Camera+ lightbox. That's actually awesome as all the edits you've done are non destructives so you can always go back to the original shot, re-edit and save it as another copy! It is like working with raw files inside Lightroom/Aperture!
There are still features I would like to see coming in like I'm personally missing a tool to straighten photo horizon but in overall I highly recommend this application to everyone into iphonography.
Have a look at some of the shots I have done while in France with my iPhone and this app (click to enlarge):
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