The app I use to look at all things light related when out on a shoot is called Sun Surveyor. The most impressive tool I use (out of the huge amount of capability this app has) is the live view on location - I can open up the app, point my camera at a building, and it shows me the path of the sun throughout the day, and exactly what time the sun dips beyond the horizon.
It's a paid app, but totally worth the $10. If you're an architect, builder, or photographer, this is invaluable.
