I've just discovered Morgue Files, described as the "Public Image Reference Archive." Content is uploaded for public use - for free - and they only thing they ask is that the photographer is credited. These are usually high resolution images, something that isn't the norm on the internet - not for public use that I've found.
Seems like a useful site to know about. Goes along with the awesome Smithsonian Images and the Library of Congress' Flickr photostream (though not all of the last two are in the public domain).
(photo by Zach Carter)