The United States Department of Energy's website on wind power in America. When reading their maps, use the 10 or 30-meter wind speeds if possible since they better represent the height of most small turbines. There will generally be more wind at 50 meters, giving an overly optimistic result if only a 50-meter map is available.
The National Renewable Energy Laboratory's wind power resource page. This image can be used as a legend for the individual state maps, as the numbers are wind power density, not speed.
