An American academic at Georgetown University, Kalev Leetaru, has started amassing what will ultimately be 12 million historic public domain images.
Located on flickr, the Internet Archive Book Image currently has 2.6 million images–photographs, graphics, maps, music, advertisements, bits of illuminated manuscripts–all downloadable copyright-free. The images date from ca. 1500 to 1922 (when copyright restrictions begin). The images are also searchable.
Some examples I found interesting on the first few pages of the archive. I have a feeling that I’ll be perusing this site regularly.