"Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license."
Contains about 75% of the Print and Photography collections at the Library of Congress.
Most of the collection can be used under fair use guidelines. However, as the LOC does not hold copyright on most of the collections, check this link for more information.
"NYPL Digital Gallery provides free and open access to over 700,000 images digitized from the The New York Public Library's vast collections.
Some of the images in the Digital Gallery may be subject to third party rights such as copyright and/or rights of privacy/publicity. Before using any of these images please review the Library's Terms and Conditions.