If you are looking for images, video, and audio, here are some good starting points:
- Finding Public Domain Images for Multimedia Projects, from EdTechTeacher. Note that some of the links on this page are out of date.
- Public Domain Material: Images, Audio, Scores & Books from Ithaca College
- Public Domain Collections: Free to Share & Reuse from the New York Public Library
- The British Library Puts 1,000,000 Images into the Public Domain, Making Them Free to Reuse & Remix, from Open Culture.
- The Metropolitan Museum of Art makes 400,000 iconic works available for download
- Library of Congress Photos on Flickr
- Selected Internet Resources — Government Resources for Science Images and Video, from the Library of Congress
- Official U.S. Government Photostreams on Flickr
- FedFlix (US Government movies)
- Search for photos on USA.gov
- National Park Service Digital Image Archive
- Freesound Creative Commons Archive (audio)
- Free YouTube Music and Sound Effects
- Creative Commons Search
- Public domain image resources list on Wikipedia Help