The Commons at Flickr. Their high resolution offerings feature photographs by Ansel Adams, Mathew Brady, lots of images from the EPA's DOCUMERICA project (1971-1977), and many more historical photographs and documents. You can view their collections here. As with all images in The Commons, these images carry no known copyright restrictions.
Image: Erik Calonius, Interior of Graffiti-Marked Subway Car, 05/1973. From the DOCUMERICA project. U.S. National Archives. From Flickr, no known copyright restrictions.