iOS: Your mobile phone camera is best for those must-capture moments in life, which are often fleeting. If you've lost one too many pictures waiting for your camera app to load (or waiting for it to save), QuickCam is a great alternative.
Instead of focusing on all-around powerful camera features, QuickCam's goal is simple: take pictures and take them quickly. The app loads almost instantly, and takes and saves pictures a bit faster than the default Camera app. It also has a great "burst mode" feature that lets you hold the shutter button to take a series of pictures in rapid succession. Alternatively, you can switch between video and still mode instantly, or even take video and stills at the same time. It also has quick access to other tweaks, like focus and exposure, with just a tap or two on the screen. If you're trying to capture something that only happens for an instant, this app's going to make it a bit easier.
I probably wouldn't make it my default camera app — I'd leave that to something with more advanced features like previously mentioned Camera+ — but this makes a great secondary app for those situations where you need to take a difficult picture quickly.
QuickCam is a $1.19 download for iOS. Note that it doesn't currently work very well on the iPhone 3G.