There have been an amazing number of historical images released for public viewing online. Here are just a few of the ones I have discovered recently:
The New York Public Library has released over a hundred and fifty years worth of restaurant menus that date from 1851 to 2008.
Yale University has launched an archive of over 170,000 images documenting the Great Depression. There is a informative article on the photos with links at Open Culture.
At Monoskop, a site for the study of arts, media and the humanities, there is a collection of avant-garde and modernist magazines from 1890 to 1939 available online.
Want to see what the Google Books project would look like if it scanned images? At Flickr, there is an archive of over 2 million images, all in the public domain, The photos, from books, magazines and newspapers, span a 500 year period and were originally scanned as part of the Internet Archive project. Eventually, the archive will encompass over 14 million images.
Image from the Internet Archive: Image from page 327 of “Handbook of the trees of the northern states and Canada east of the Rocky mountains. Photo-descriptive” (1907)