Public Domain Media

FedFlix: Over 3,000 public domain U.S. government films in their entirety.  Includes nuclear test footage, NASA films, public service announcements for health and hygiene, cold war films, etc. Films may downloaded free of charge.

Flickr Creative Commons  Different types of licenses are available, including Attribution,  Noncommercial, Share Alike, and Public Domain.

Museo Museo is a visual search engine that connects you with the Art Institute of Chicago, the Rijksmuseum, the Harvard Art Museums, the Minneapolis Institute of Art and the New York Public Library Digital Collectionmore to come! Every image you find here is in the public domain and completely free to use, although crediting the source institution is recommended

Internet Archive Book Images: "An archive of 2.6 million public domain images, all extracted from books, magazines and newspapers published over a 500 year period."

Library of Congress Photos on Flickr:
About 3,000 photographs (in color and black and white) and posters from governmental sources such as the Library of Congress, the FBI, National Archives and Department of Defense. Includes portraits of politicians, celebrities, protests, demonstrations, and news shots from around the world. No known copyright exists for the photos included within the collection

LibreStock
LibreStock is a metasearch engine that searches over 40 CC0 image websites.metasearch engine that searches over 40 CC0 image websites. Contains over 60,781 high quality do-what-ever-you-want stock photos.

Newspaper Navigator (In Library of Congress' Chronicling America) Search 1.56 million historic newspaper photos using Newspaper Navigator.

Pixabay
"On Pixabay you may find and share images free of copyrights. All pictures are released under Creative Commons CC0 into the public domain.
You can copy, modify, distribute, and use the images, even for commercial purposes, all without asking for permission or giving credits to the artist. However, depicted content may still be protected by trademarks, publicity or privacy rights." 
Take note:
"Those images marked with the Shutterstock logo are shown to finance Pixabay and to provide a choice of professional photos."

Unsplash
Over 300,000 free (do whatever you want) high-resolution photos brought to you by the world’s most generous community of photographers.

Wellcome Images: Covers"2000 years of human culture". Images on this site are freely available for download for personal, academic teaching or study use, under one of two Creative Commons licences.

Wikimedia Commons
Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone, in their own language 
Unlike traditional media repositories, Wikimedia Commons is free. Everyone is allowed to copy, use and modify any files here freely as long as they follow the terms specified by the author; this often means crediting the source and author(s) appropriately and releasing copies/improvements under the same freedom to others. The license conditions of each individual media file can be found on their description page. The Wikimedia Commons database itself and the texts in it are licensed under the Creative Commons Attribution/Share-Alike License