I'm thinking of compiling a map showing how often it was sunny at a given location on a given day of the year, one map for every day of the year. The exact definition of "sunny" will depend on what cloud cover data I can obtain.

Are any such data sets freely available? I've found MODIS image archives, but they are fairly limited, especially in that they are raw images, not interpreted as cloud cover. I've also found a company that seems to sell exactly what I want, but I don't expect this to come cheap.

Can I get historic cloud cover data for free?