Materials in the public domain may be copied, reused, shared, or distributed without permission from the creator or paying a fee. Government documents or works created by an office or employee of the federal government are automatically entered into the public domain whereas commercially published and privately created works enter after a statutory period. Commercially published and privately created (unpublished) works will enter the public domain after a statutory period has passed. This statutory period varies depending on when the item was published. In general, anything published before 1923 is in the public domain.
The ALA Office of Information Technology Policy created a graphical slider to determine whether an item is protected by copyright (requires Flash).
Cornell University has created a table titled “Copyright Term and the Public Domain in the United States.” It is updated annually.
The Commons on Flickr is a public photography archive. It contains photos from participating institutions such as the Library of Congress, the Smithsoinian Institution, and NASA. The photos in the Commons have "no known copyright restrictions."