Flickr contains many images that are CC-licensed or in the public domain. First, search for your topic, then click on Any License at the top of your results. From there, you can select different options for how narrow you want your results.
The Internet Archive is an excellent source of public domain works and CC-licensed work. Make sure to check the record before you reuse any audio. You should see a CC license icon or a C with a line through it (the public domain mark). The example below shows both icons.
Vimeo hosts many Creative Commons videos. Search for a video on your topic, and then select More Filters on the left. You'll then be able to limit your results by CC license. If the original uploader has allowed it, these videos will include an option to download.
The Digital Public Library of America has over 7 million items. To locate films, search for your topic and then select moving image from the filters on the left. Rights information is listed in each item's record.
Google has digitized many books that are in the public domain. To limit your search to books that Google knows are in the public domain, limit your search by adding date:1790-1922 either before or after your other search terms.
The Internet Archive is an excellent source for public domain and Creative Commons works.. If you find a book here, make sure to check the record for rights information. Look for a Creative Commons license or the phrase NOT_IN_COPYRIGHT (for public domain works).