Live by the cloud, die by the cloud. Ok, life as a photographer isn't that dramatic, but knowing what the sky is going to do is great information to have. Clear Dark Sky gives you that information and more.
Although designed for astronomers, Clear Dark Sky is equally useful to any photographer who work outdoors, particularly those who work with natural light or astrophotographers. Its interface is very simple; just enter your location and select the chart location you'd like to use. You'll be presented with a chart like this: