Open Educational Resources: Public Domain

Works in the public domain are not protected by copyright and may be freely used by everyone. Once a work enters the public domain, it cannot be reclaimed by the original copyright owner. Reasons that these works are not protected include:

• Work does not meet the requirements for copyright.
• Copyright for the work has expired.
• Author/creator failed to satisfy statutory formality to perfect the copyright.
• Work was created by the federal government (does not apply to works by other government entities, grant recipients, or contractors).

The American Library Association's Office of Information Technology Policy has created the Public Domain Slider tool to help you determine the copyright status of a work that is first published in the United States, and Cornell University also offers a chart to help determine what works are within the public domain.

Government Records are public domain materials.

Wikipedia: Public Domain Resources
comprehensive list and links to public domain materials for a variety of subjects 

Public Domain Review: Collections
links to global public domain collections from a non-profit established in 2011 dedicated to works from the history of art, literature, and ideas

Library of Congress
huge collection of American print, pictorial, and audio-visual material with most in items in public domain

National Art Gallery
images of works of art in the collection deemed as public domain

Project Gutenberg
the first provider of free electronic books with a top collection of public domain literature and written works

PDPoster
collections of public domain posters