The Library of Congress has one of the largest archives of any library in the world. Over the last decade the LOC has made a major effort to digitize its collection and make it accessible to the world. I recently came across an incredible collection that is free for "use and reuse". Although they urge users to consult the rights statement for each set they are essentialy copyright-free for either private and commercial use. Here is the libraries introduction to the sets. Follow the links and view them yourself. I particularly admire the "Japanese Prints" set.
Free to Use and Reuse Sets
This page features items from the Library's digital collections that are free to use and reuse. The Library believes that this content is either in the public domain, has no known copyright, or has been cleared by the copyright owner for public use. Each set of content is based on a theme and is first featured on the Library's home page.
These sets are just a small sample of the Library's digital collections that are free to use and reuse. The digital collections comprise millions of items including books, newspapers, manuscripts, prints and photos, maps, musical scores, films, sound recordings and more. Whenever possible, each collection has its own rights statement which should be consulted for guidance on use. Learn more about copyright and the Library's collections.