I find myself going back to this site over and over again. It has historic aerial photos, some going all the way back to 1938. Depending on where you live, you can see how your area looked over many years.
Here's a fun site featuring old aerial photos of rural areas, farms, and small towns. It's a work in progress, and some counties are searchable while others are not. If you see a familiar place, you can leave a comment and pinpoint the location on a map for future searchers. The photos are also available for purchase.