http://www.historicmapworks.com/
Discovered the site by happenstance on Sunday when I was googling for something else.
Alas, they have alot of maps...of some regions (Ohio & Penn - good coverage. Miss & Louisiana - no coverage) with county maps from the end of the Civil War up through 1980.
Interesting to look through if you're interested in looking at city and highway development.
I had subscribed to that site a couple years back. I then discovered that after initially poking through quite fascinatedly, I wasn't using it, and I let it expire.
It's no longer subscription-only, but who knows if they'll change it back. http://wardmaps.com/ is a similar site.
I think this may have been linked here before, but Historic Aerials (http://www.historicaerials.com/) has some excellent aerial images of more populated areas, mainly dating from the '50s to today.
Historic Aerials is very interesting. In the Detroit area, you can get a glimpse of the old interchange at I-94 and Telegraph, the old one at I-94 and Southfield, Spaghetti Junction before it was Spaghetti Junction, and the Jeffries under construction.
Love being able to see Ebbets Field on Historic Aerials.
Today's addition: For covered bridge lovers
http://www.handsomeproductions.com/tfkeller/index.html
Color photos of Covered bridges from the 1940s, 1950s, and maybe 60s.
Geographic focus is Eastern Great lakes and Northeast.