I've been using 'The Best Camera' iPhone app for a couple of weeks now and I have to say that it's the best photography app that I've came across.
The app is by photographer Chase Jarvis, who is one of the online photographic communities most prolific contributors. Chase has also put out a book of his iPhone photos called 'The Best Camera Is The One That's With You', and put up a website that ties in with the app. Here's how it works:
You take a photo with your iPhone or use one that's already in your Camera Roll folder. Then you apply any of the cool filters that do things like - add saturation, desaturate, darken, warm-up, cool-down, add vignette....etc. You can also crop the photo into a square and add a white border. The filters can be moved into a different order to change how they effect the photo, or you can remove the ones you don't like.
After you're done with the editing side, you click on the Share button and you can save it back to your Camera Roll, send as an email, send to Facebook and Twitter (adding text to your tweet too) and even upload it to thebestcamera.com to be displayed on the site. Tap on the Globe icon and it will take you to a viewing wall (see the photo above). This is the view from the Best Camera website and is constantly changing. Now you can see your newly uploaded photo and you can click on a thumbs up icon if you like other peoples photos. Warning it's addictive...but it's cheap!