The Library of Congress has a huge collection of public domain photographs from various collections, including the FSA (Farm Security Administration), the OWI (Office of War Information), and collections from the civil war and the HABS/HAER (Historic American Building Survey/Historic American Engineering Record). The image quality is uneven, but there is some great stuff there.
Apparently, the Library of Congress is now getting involved with Yahoo and they're posting some of their photographs to Flickr: http://www.flickr.com/commons.
In addition, an organization called publicresources.org is posting images from the Smithsonian Institution that seem to be in the public domain: http://www.flickr.com/photos/publicresourceorg/collections/.
Update: Patrick Pecatte alerted me to another great collection of public domain photographs, PhotosNormandie, which has some heart-wrenching images from France during World War II. I'm not sure about how these photographs originated, but Patrick invites people to contribute accurate information about them.
More good ways to waste time!