For the most part they are all SCAMS. I have encountered very few however that have delivered on their promise. It's usually the get-paid-to sites that are the most reliable but a few others are as well.
I have come across countless websites claiming to offer free PSN codes but all of them turned out to be nothing more than SCAMS! I am very annoyed that someone would go to all that trouble to create some robust fancy looking website just to trick people!
It would be a miracle if someone actually created a working PSN code generator but I seriously doubt such a thing would ever happen.