Works fall into the public domain for three main reasons:
1. the term of copyright for the work has expired;
2. the author failed to satisfy statutory formalities to perfect the copyright or
3. the work is a work of the U.S. Government.
As a general rule, most works enter the public domain because of old age. This includes any work published in the United States before 1923. Another large block of works are in the public domain because they were published before 1964 and copyright was not renewed. (Renewal was a requirement for works published before 1978.) A smaller group of works fell into the public domain because they were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).
Many public domain resources can be found on the web.
Creative Commons is a nonprofit organization that works to increase the amount of creativity (cultural, educational, and scientific content) available in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing. A searchable database of diverse media can be found at Creative Commons.