Does anyone know of any websites that feature open source MFC addins for things around UI?
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            You can search www.codeproject.com for a lot of different UI libraries. There is a lot of work available there which is absolutely free and(sort of) open source.
Check this link for a list of MFC related articles. Although all of these are not add-ons but some of these are.
If you can pay some money then BCG Library is certainly one of the best options available.
                  Aamir
                   2009-05-04 10:29:36
                
              
                
                A: 
                
                
              
            Probably the best free one. http://www.codeproject.com/KB/MFC/#MFC%20-%20The%20Ultimate%20Toolbox
                  Gadu
                   2009-05-04 11:06:59