National Solar Radiation Database
Ends 2005, the solar radiation is modeled from 1400 stations with measured data from only 40 stations.
NASA Power Site
Offers data based on satellite images for almost any location. Click "Agroclimatology."
Modeled from satellite images and limited number of ground stations.
National Renewable Energy Laboratory
NREL has an attractive solar atlas based on modeled data as well as other GIS products.
There are detailed statistics comparing us to the others here.