I've seen a few iOS apps that allow you to take photos in dim lighting conditions, but none so far are as well thought out or feature rich as NightCap Pro (US$1.99).
The app, which supports both still photos and video, puts your camera into enhanced modes that really improve low light performance. NightCap Pro also features grain reduction, always a problem when shooting in low light, and a Light Trail Mode for getting good images of light painting or even star trails.
The onscreen displays are extensive, and when you first use the app you'll get a tour of what the icons mean. You can lock exposure, focus (critical at night) and white balance. There is a self timer and also a burst mode, with adjustments for the amount of noise reduction. Add too much and the image gets soft, so having an adjustment is a good thing. Photos are saved at the full resolution of the camera for stills. For video, resolution is lowered for Night Mode.
I gave the app a try in my generally dark neighborhood. We have no street lights overhead; just exterior house lighting. The photos (see the slide show below) looked quite good. I could even see some bright stars in the image, not something I am used to seeing in an iPhone shot. I didn't try star trails, but I've seen some samples taken with the app and they were impressive.
No camera can shoot in total darkness, as there has to be some ambient light to trigger the sensors. But NightCap Pro does an excellent job of letting you shoot photos in lighting conditions that you'd normally avoid. There is plenty of control flexibility, and colors remain natural, as opposed to the green cast that some low light apps provide.
NighCap Pro requires iOS 7. It is a universal app, but expect the best performance on the latest iOS devices.