The U.S. National Archives have become a member of the Flickr Commons. To join the Commons, members must "claim 'no known copyright restrictions' on the content they share." Here's the National Archives' photostream.
Here's an excerpt from the press release:
To mark the opening of its photostream in the Commons today, the National Archives is posting a new photo set containing more than two hundred photographs of the American West by renowned American photographer Ansel Adams. The photographs, taken between 1941 and 1942 as part of a Department of the Interior mural project, feature the Grand Canyon, Yellowstone, Glacier and Zion national parks, in addition to Death Valley, Saguero, and Canyon de Chelly national monuments.
The Ansel Adams photographs join a larger selection of more than 3,000 National Archives images that are part of the National Archives' Flickr photostream. The photostream contains a variety of images from some of the National Archives most popular collections, including images of the Civil War by Mathew Brady and associates; images from the Environmental Protection Agency's 1970s photo-documentary project DOCUMERICA; images from the Records of the Women's Bureau depicting women in the war labor effort during World War II; and a grouping of favorite photos and documents from the National Archives, featuring among others the 1970 photograph of President Nixon shaking hands with Elvis Presley.