If you were one of the brave few to begin jailbreaking your iPhones back in early 2008, you probably came across what I considered the best app available at that point: Snapture. I always wanted to get more out of my iPhone camera and this jailbreak app helped me accomplish that.
Fast forward a year and a half and you'll finally find Snapture [iTunes Link] in Apple's App Store. This is primarily due to the fact that Apple has opened up more and more of the iPhone OS with each new release of the SDK. Is it perfect? No, but with the given access to the Camera APIs, applications like Snapture and Best Camera can finally exist in a world of harmony with the big wigs at Apple.
In regards to functionality, I've always been a huge fan of Snapture. It's simple, but it works. I don't have to touch a button on the screen to take a picture, I can tap the screen anywhere I'd like and it will work just fine. My biggest pet peeve about the built-in camera app is that I have to press a tiny button in order to take a picture. That's not very conducive for self portraits or any other situation where you're not looking right at the device.