Wow! That’s the reaction I had to seeing the New York Public Library’s photo archive online for the first time. Over 270,000 images in the digital version of the NYPL’s photo archive. Amazing.
The City of Houston has a photo archive as well that is pretty amazing itself. Unfortunately, it is not online, but is well-kept in the libray annex (Julia Idelsen Building) across from the main library building downton on McKinney. I highly recommend it if for no other reason than the history contained in the over 3 million images stored there.
The fact that you can get actual photo reproductions for very little money helps too.