Shorpy is a great source for old photographs. They often get them from sources like the Library of Congress which is where this photo of the New Cannan public library in 1953 came from. You can also see the original set of photos over at the Library of Congress [did not see this one over at their Flickr photostream]. The big add that Shorpy’s has, however, is the community. It’s not just a photo of a library, it’s also people commenting about their memories of the library including where else they’ve seen that certain floor tile [fun fact: it's also the tile that's in my bathroom as near as I can tell] [thanks mike]
I’m using the Library of Congress version of this photo of the reading room at the University of Michigan library (which has not made its way to Flickr yet) but I found this image via Shorpy. It’s a nice non-Flickr illustration of how having a way to have users comment on your content can increase everyone’s level of knowledge. The first comment has a link to this web site which contains another photo of the same room, highlighting the statue, and the story of what happened to it.