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).
Use the Copyright Slider Tool to determine is a work is still protected by copyright.
Search the CREATIVE COMMONS.
Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools. Search for images, videos, and other materials that are shared by authors for academic use.
Search MERLOT - Mulitmedia Educational Resources for Learning and Online Teaching
MERLOT is a free and open peer reviewed collection of online teaching and learning materials and faculty-developed services contributed and used by an international education community.
Search Khan Academy - a non-profit educational website created in 2006 by educator Salman Khan to provide "a free, world-class education for anyone, anywhere.
Search MIT Open CourseWare - Video/Audio Lectures, lecture notes, online textbooks, interactive simulations, and assessments. Freely accessible and reusable under the Creative Commons license.
Search Open Educational Resources (OER) - including textbooks, videos, lesson plans, lecture notes, syllabi.