Planning photographic sessions "on location" and estimating the Sun position is a tricky process. Up until now I've used a Solunar Table and Digital Compass. They are reliable but a little slow to use in the field. Roll on the "Focalware" application from Spiral Development. It cleverly uses the iPhone 3Gs digital compass and GPS unit to locate where you are and where you are pointing the phone. It displays a live position of the Sun against those details and can even predict where the Sun will be with the simple drag of your finger across the dial. This saves a ton of time and aids in much faster visualization.
Recently at a wedding, in the heat of an Austin summer, the Sun was threatening to cut the scene in half with strong light and shadow. When the bride asked if a 10 minute delay would be okay I responded "yes". I had determined from a solunar table, compass, and a nearby building that the extra time would fully cloak the scene in shade making the pictures more flattering and the client cooler. Focalware would have allowed me to check this instantly and any additional angles without fumbling with paper and compass.
If you photograph outdoors then you need this app and an iPhone 3Gs!