I love historic aerials.com. You can browse old photos for free, but need to buy individually or sign up for one of their various subscription plans to just download them (and get rid of the annoying watermarks.)
Some areas have better aerial coverage than most, but most urban areas seem to have photos every few years from 1930 on. There is also the slider bar that allows now and then comparison side by side.
I waste almost as much time there as I do here......