Many of the returns of the early US imagery reconnaissance satellites have been declassified. These satellites exposed panchromatic film on orbit that was dropped in canisters that were caught by aircraft. The resolution of the system is incredible: a single frame can be over 1000 megapixels.
Scans of the original negatives are available from USGS through Earth Explorer. These are free to download for negatives that have already been scanned in, and it costs $30 to have a new negative scanned in. After that, it becomes publically available. I requested a shot of Seattle, so we'll see how long that request takes.
It's a bit inconvenient to view an image this large, so I set up OpenSeadragon to display some images here. The image below is a 1,237 megapixel shot of St. Louis, MO taken on March 12, 1964 during mission 4006 of KH-7 Gambit. Note that the arch, several interstates, and the Poplar Street Bridge are under construction.
Shot of St. Louis, MO taken on September 23, 1966 during mission 1035 of KH-4A Corona. Note that the arch is now up, Busch Stadium II is operational, and the Poplar Street Bridge is halfway across the river. On the Illinois side, you can see the Granite City Steel Mill discharging a huge amount of smoke.
This last image was taken by an airplane. It doesn't cover as wide an area, but it is higher resolution and shows off Pruitt–Igoe nicely. Pruitt–Igoe was an infamous public housing complex built in the early 1950's and demolished in the early 1970's. To bring this full circle, the federal government is constructing the new western headquarters of the National Geospatial-Intelligence Agency across the street from the still abandoned Pruitt–Igoe site.