Items in the public domain are those that are not under copyright, usually due to their age. Because there have been many changes in copyright laws, it can be difficult to determine whether something is in the public domain.
For an overview of US Copyright rules, read the Duration of Copyright: Provisions of the Law Dealing with the Length of Copyright Protection summary from the US Copyright Office.
For more detailed and official information about the copyright status of film and television works, please consult the website of the United States Copyright Office. The website has a wealth of information concerning copyright, including a comprehensive list of frequently asked questions, a complete collection of their publications, and registration forms. There is also an online database of registrations and renewals since 1978.
Free to Use and Reuse: Public Domain Films from the National Film Registry
The National Film Registry selects 25 films each year to showcase the range and diversity of American film heritage. These public domain selections are part of an online collection.
Learn more about this selection of films from the National Film Registry.
Provides near-unrestricted access to digitized collections of moving images. The largest collection comprises over 1,200 ephemeral (advertising, educational, industrial, and amateur) films made from 1927 through the present. Broadcast quality copies can be purchased through Getty Images.
PublicDomainTorrents.com
Public Domain Movie Torrents hosts a wide variety of movies now in the public domain.