I like HistoricAerials, but sometimes find it lacking for older photos (pre-1950s), at least in areas i've researched. If you're looking for recent history (say 1990-current), Google Earth usually has more frequently updated aerials.
For much older research, I have luck with governmental resources (DOT, county gov't websites). If you're in a region with a large University, they may be very helpful. For example, University of Buffalo has a number of aerials for Erie County.
Hope that helps.