Political Database of the Americas - The mission of the Political Database of the Americas is to contribute to the study, promotion and strengthening of democracy in the hemisphere, through the collection, systematization, dissemination, and exchange of information, data, statistics, and institutions from a political perspective, comparative studies; and other resources relevant to the 35 countries of the region.
Digital Public Library of America- The Digital Public Library of America aggregates more than 14 million items from libraries, archives, and museums. Items include text, videos, images, and audio.
New York Public Library Digital Collections - The New York Public Library has made almost 700,000 items - mostly images, although text, maps, audio, and movie files are included - in their digital collections free to the public. Check individual items for any other copyright restrictions.
USA.gov - Do a simple search or browse through videos and images from the federal government.
The Public Domain Project - Hosted by the stock footage company Pond5, this website curates almost 100,000 image, video, audio, and 3D model files in the public domain.
Smarthistory - Focused on art and art history, this website includes images and videos based on geography, time periods and styles, and themes.
Library of Congress - About a dozen collections that include free-to-use and reuse material.
Images
Open Clipart- an online media collection of more than 160,000 vectorial graphics, entirely in the public domain.
Getty Images - Getty Images has more than 100,000 images that are considered open. This search page is set up to include only those images.
Wellcome Images - These images from the Wellcome Trust have a focus on current and historical medical topics.
Google Images - This advanced search page for Google Images allows users to search for images that include a Creative Commons license. Use the last filter on the page, Usage Rights, to set your terms.
Flickr - Like Google images, Flickr also allows you to search for images that come with a Creative Commons license. This page is set up for that already, but you can also filter any search by click on "any license" in the upper left corner of the page to choose what license you want.
Visual Hunt - Millions of images that have been licensed with a Creative Commons license.
Smithsonian - Includes 2.8 million images from the Smithsonian's collection that have been released in the public domain.
Pexels - Stock photos that have been placed in the public domain.
Pixabay - Another site of stock photos, vectors and illustrations placed in the public domain.
Audio
Internet Archive - Music - Thousands of audio files curated in the Internet Archive. Includes all types of audio files. Check to see if specific ones are licensed to be adapted.
Free Music Archives - Music that's been cleared of copyright issues. Browse by genre or most popular.
University of Oxford Podcasts - Open podcasts made for University of Oxford classes. Browse through by topic or do a simple search.
BandCamp - These music files uploaded to BandCamp include a Creative Commons license.
Freesound - These audio files focus more on snippets, sounds, bleeps, etc. Includes searching by a "sounds like" feature and a help forum to make specific requests. Files should include a Creative Commons license.
Videos
The Open Video Project - These free videos are curated by the University of North Carolina at Chapel Hill but have not been cleared of copyright issues. Many of them come from the federal government.
Internet Archive - Movies - This collection features almost 3 million videos archived by the Internet Archive. Check individual collections and movies for copyright status.
YouTube - You can specifically search for videos uploaded to YouTube with a Creative Commons license. After you run a search, click the Filter box at the top of your results list. From there, in the Features column, select Creative Commons.