Most of my photos are taken with my iPhone (currently an iPhone 4), because it the the best camera I own—they say the best camera is the one you have with you. I also own a Canon S90, which is an excellent camera. It’s got just the right balance between portability and advanced features like a lens with f2.0 aperture (although the sensor is small, so the low-light functionality isn’t as great as I had hoped).
Besides the iPhone camera access from the lock screen, my favorite camera apps are black and white camera app Hueless (Hueless photos | App Store), and the panorama app 360 Panorama (360 Panorama photos | App Store).
Hueless: black & white camera app
360 Panorama: Panorama app