What is Public Domain?
The public domain includes all of the creative works not covered by copyright. It also includes most works created by the United States government.
Works in the public domain are free for the public to use! These works can be copied, modified, or used in any way without asking permission.
A work will enter public domain in one of these ways:
• When copyright expires
• When the work was created before 1923
• When works have been assigned to the public domain by their creator
What works have expired into the public domain?
Congress has passed a series of laws extending the term of copyright. Currently, the default term is life of the author plus 70 years. That means that most of the copyrighted works created from the late 1970s to the present may not become public domain during your lifetime.
In general, works published after 1977 will not fall into the public domain until 70 years after the death of author, or, for corporate works, anonymous works, or works for hire, 95 years from the date of publication or 120 years from the date of creation, whichever expires first.
So what works have expired?
• All works published in the United States before 1923
• All works published with a copyright notice from 1923 - 1963 without copyright renewal
• All works published without a copyright notice from 1923 - 1977
• All works published without a copyright notice from 1978 - March 1, 1989, and without subsequent registration within 5 years
Source from Cornell University, "Copyright Term and the Public Domain in the United States"
Where do I find public domain works?
Photographs/Images:
Public Domain Pictures, a repository for public domain images
The New York Times Public Domain Images, all images are from the New York Times archives
Music/Audio
The Internet Archive - Audio Archive, contains digital recordings
Public Domain Information Project, provides a list of public domain songs and sheet music
LibriVox, public domain audio books
Films
Prelinger Archives, all public domain films from this archive are available for download and use
The Internet Archive - Moving Image Archive, over 200,000 free digital recordings of concerts, radio shows, news programs, book and poetry readings, and original music uploaded by users
Text
Project Gutenberg, digitized books in the public domain
The Internet Archive - eBooks and Texts, over 8,000,000 public domain eBooks