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Science.gov: Select "Multimedia - Multimedia sources" on the advanced search page to search videos and photographs from U.S. government sources such as Library of Congress, MedlinePLUS, NASA, NOAA, and NSF.
Wikimedia Commons: Contains many scientific illustrations, photographs, and animations submitted by users. Contents can be browsed by topic or searched by keyword. The license conditions of each media file are given on its description page.