Have you ever browsed around on the Library of Congress Prints and Photographs page? Do it. Right now. You can search all the collections, or hone in on one specific collection. In most cases, you can not only view a thumbnail of the image, but you can also download a high-quality JPG or even an archival quality TIFF file. In case it isn't readily apparant, this is really really awesome.
Every collection is pretty neat, but I particularly like the Bain Collection, the Carpenter Collection, Historic American Buildings Survey, National Photo Company Collection, Photochrom Prints (which I love because of the awesome colors, and because you hardly ever see color photos from this time period), and Popular Graphic Arts. I could go on, but my fingers are tired of typing.
[Sometimes it can be hard to guess at what search terms are available in a collection, so I think its easiest to go to Browse: Subject headings, and just start looking around from there.]