Night Camera HD features exposures up to one second, a manual ISO3200 setting for really dark locations, an on-screen histogram, tap-to-focus and automatic or manual exposure adjustment. There's a handy timer, so you aren't moving the camera when the shutter engages. In fact, for most night photography, you'll want the camera on a small tripod or held VERY steadily against a solid object.
I tried Night Camera HD around my neighborhood in minimal light, and inside dark rooms. The app worked well. In a mixed-lighting environment (a dark room with blinds closed, but some bright sunlight leaking through), the app produced a presentable image, but so did Apple's Camera app via HDR, something that's not available on Night Camera HD. I did notice the Night Camera HD image had lower noise.
Night Camera HD features on-screen grids for composition and keeping the horizon level. It works in landscape and portrait modes, and outputs photos at full resolution. It also allows you to use the volume control buttons on the iPhone. The volume up button triggers the shutter, and the volume down button allows you to hide some of the on-screen GUI. Be careful when exposing a low-light image with a volume button, though, as you're likely to move the phone and produce a blurry image.
Night Camera HD is a handy addition to your phone. It's a good value for $0.99. It requires iOS 5 or later, but some features will require the newest iPhones, such as the fast ISO3200 mode.
The app is not universal, but there is an iPad version with identical features. There are other apps that allow for long exposures, but this app specializes in low-light photography and has the features to help you get good results.