There are many jailbreak tweaks that allow you to map the iPhone's volume buttons to the camera shutter. As far as I'm concerned, though, none of them have as much potential as CameraButtons.
CameraButtons takes the essential functionality of past tweaks, and ramps it up a few notches. Not only can you control your shutter with the iPhone's physical buttons, but you can also map a plethora of other camera-based functions to them as well.
If you're an avid user of the iPhone's stock camera, then you're going to want to check out this video of CameraButtons in action...