A great iPad app that lets you select your location (by GPS or manual location finding) and will show you where the light is coming from at any part of the day or moon at night. This allows you to gauge whether a particular location is suitable for a shoot given the angles.
http://digitalphotobuzz.com/ipad-app-review-lighttrac
There is a similar app for the iPhone made by another company:
http://photoephemeris.com/
This website also has a Mac, Windows and Linux desktop version for you as well - nice
