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:"Cloud Cover" is just that, with grays and whites corresponding to varying levels of overcast skies, and darker blues corresponding to clear skies. "Transparency" measures how water vapor in the air affects the ability to see space objects. "Seeing" measures how air turbulence affects the ability to resolve fine details of objects in the sky. "Darkness" tells how dark the sky will be at that time; it takes into account the sun and moon, but does not consider light pollution. I've used Clear Dark Sky's charts before and have found the cloud forecast to be quite accurate both during the day and night. The more astronomically oriented parameters are quite accurate and helpful as well. Try it out here.