Archive from the Library of Congress, includes many historical photographs and film stills.
Flickr: Creative Commons
Search for images on Flickr released for use under a Creative Commons license.
Suggested attribution instructions for images from Flickr
Liam's Pictures from Old Books
Almost 3,000 high-resolution images scanned from publi domain books. Includes illustrations, borders, decorative elements, and alphabets.
Stock photography provided free for all uses, including commercial.
NYPL Digital Gallery
Images scanned from the vast collection and archives of the New York Public Library.
Public Health Image Library (PHIL)
US Dept. of Health and Human Services Centers for Disease Control and Prevention provides a searchable database of photographs, micrographs, and illustrations relating to public health.
Many sites require you to create an account before you can download files for use. Watch for the usage permissions, whether the video may be used in only non-commercial presentations, commercial presentations, or both. Some sites require you to purchase a subscription, or to purchase clips you download. It is important to understand the copyright and licensing laws behind the use of video and audio files. Royalty-free means that there are no additional costs/fees associated with the audio/video files beyond the initial fee to use the file. Many sites of license agreements that users must agree to in order to purchase and download the files.