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.
http://digitalgallery.nypl.org/nypldigital/index.cfm
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.