OpensolarDB is a database with average global daily solar radiation data for each month of the year as measured on the ground. The uniqueness of this solar database is that it is open for everybody, open for extracting data and open for introducing data. Everybody can extract data. This extracting can be done in various ways: by downloading, doing a copy/paste on-line, by connecting to a webservice or by Flex Remote Object services (amf format).
The solar radiation data in OpensolarDB are free and very accessible in many ways.
OpenSolardb website