The Photographer's Ephemeris is a fantastic little application that provides a wealth of useful data for photographers about the sun and moon.
Simply enter your location and the program gives you a ton of information including sunrise, sunset, moonrise, moon phases, and when and where it's all going to happen relative to your location and time of day. As an architectural photographer, knowing where the sun is going to be at any given time of day is extremely important.
The application uses Google Maps so a precise location can be pinpointed and the whole thing is very easy to understand and visually oriented so that you don't have to be an astronomer to figure out what's going on (good for us photographers!).
Word is that there's an iPhone App coming soon...