For years I've used the Complete Sun and Moon site to decide when to schedule a twilight shoot so I can make use of that beautiful light that only occurs at for about 20 to 30 minutes at sunset and sunrise. The challenge of course is being on site, setup and ready to shoot during this magic time.
Now there is a cool new iPhone application (Focalware $9.99) that takes sun planning to a whole new level. As you can see from this demo video, in addition to telling you when sun rise and sunset will be on any given day, this application allows you to figure out exactly which direction the sun is going to be coming from at any time during the day. This is great for planning for exterior shots.