http://www.flickr.com/commons?GXHC_gx_session_id_=6afecb2055a3c52c
Content: Close to 40 national cultural heritage institutions have joined Flicker Commons to provide digital images in one location. Some participating institutions include the Library of Congress, The Smithsonian, the New York Public Library and many university collections.
Pricing: Free (non-commercial usage), but not necessarily in the public domain
Terms of Use: “By asserting ‘no known copyright restrictions,’ participating institutions are sharing the benefit of their research without providing an expressed or implied warranty to others who would like to use or reproduce the photograph. If you make use of a photo from the Commons, you are reminded to conduct an independent analysis of applicable law before proceeding with a particular new use.” https://www.flickr.com/commons/usage/