Solar radiation data free?

Yes. Lots of data, but not a lot of historical archived measurements.
Most of what's called historical data that's in the open literature and available is extrapolated from modeled data derived from 26 measurement sites around the U.S. done years ago and that data is almost exclusively of Global Horizontal Irradiance (GHI) that most be recast into Plane of Array (POA) irradiance to be of any use for solar energy power generation work.

I can elaborate but I'll need more detail about what type of data you have in mind and what you want to do and how much background you have that might help me understand how much you already know so I can perhaps point you in the right direction.

There may be some audio/visual info available but I'm not aware of any.

There also is something called the NSRDB - The National Solar Radiation Data Base, but that is NOT historical data. It's modeled. Quite good for modeling with but it is most definitely not historical data.

For more info see: WWW.nsrdb.nrel.gov. for more information on that data base and also something called the TMY - Typical Meteorological Year.

Good Luck.

The National Centers for Environmental Information (NCEI) and OpenEI are two websites where users can download historical weather data for solar radiation and other parameters for free. NCEI is the world's largest archive for weather and climate data, whereas OpenEI is a platform operated by the US Department of Energy. Several videos are available on YouTube that guide users on how to obtain data, such as the "How to Download Historical Weather Data from NOAA" video by Giselle Baumet. These resources allow users to access relevant data to analyze atmospheric conditions and plan solar energy projects effectively.

Yes, but the OP's post stated "...historical weather data".
Most all of the irradiance data is still modelled and not measured historical data as is most of the other weather data that's available.
Still, OpenEI and NCEI sources are available and useful, but the NSRDB files are the easiest to use for what the OP seems to be looking for and are easiest to get at through the NREL website.

Oh okay, It's good to know about the NSRDB files and NREL's website. Appreciate your input regarding the availability of the measured versus modeled data. Thanks!

You're welcome.
For details about how/why most of the historical and also most of the more recent irradiance data are modeled rather than actual as well as other interesting factoids, see the NSRDB handbook.

It's not necessarily downloadable and not necessary pure data in the 'data' sense, but many of the weather stations on ambientweather.net have solar data that is pretty accurate. I use my neighbor's two houses down to see how well my enphase system is doing since the graphs should be pretty close if all is well.