Outdoor photographers are going to love PhotoPills. This US$9.99 app is a treasure trove of information for photographers who do most of their work outside.
The app allows you to plan your photography for any location on earth by letting you know where the sun and moon will be, provides depth of field calculations for any of hundreds of DSLR cameras, can help you determine the number of exposures needed for time lapse videos, and even calculate how much space images will consume in your camera storage.
I especially like the augmented reality views of the Sun, Moon, Milky Way and the North Star (Polaris) so you can know exactly where to position yourself for that dramatic photo of a striking sunset or sunrise.
PhotoPills can also perform calculations for determining the best long exposure times or determine the field of view for any camera/lens combination. It even tells you when the moon is closest to us or at its greatest distance, and of course you will get a graphical indication of the moon phase.
I live in an area with many canyons, so the ability to go to a location just before sunset and see the position of the sun superimposed on the actual terrain really helps to get me in position.
You can save points of interest right in PhotoPills or share information with other photographers. PhotoPills is not an app for the casual point-and-shoot crowd, but targeted at people who want to get the most out of their photographs by getting loads of useful information before pressing the shutter button.
PhotoPills is not an inexpensive app, but it does things no other photography app does, and it performs complex calculations very quickly. If you are a pro or serious amateur photographer, then PhotoPills is for you.
A similar app in terms of sun predictions is Sun Scout, but it also costs $9.99 and doesn't have nearly the information or set of calculations that PhotoPills provides.
PhotoPills requires iOS 5.1.1 or later. It's not universal, so really it needs an iPhone with its array of sensors. It's hard not be enthusiastic over what this app can do. You can view a video to see the app in action at the PhotoPills website.