There are probably some more Flickr accounts and museum/library collections we could add to this list. See also below on searching Google Images for images without copyright constraints.
Picryl, “a public domain search & similarity engine.”
RawPixel (not all free, but much free; photos, illustrations, vectors, PSD…)
Stock photos of women of color in tech, now free to use.
nappy.co: "Beautiful photos of Black and Brown people, for free."
Broadly/Vice gender diverse stock photos
Age-positive photography from the UK Centre for Aging Better (press release)
WNYC free stock photography (modest, but fun to see news nerds in the photos)
http://www.freeimages.co.uk/ (no search index, just browse by category, some weird googlish search)
http://www.freeimages.com/ minimal non-ad results
https://morguefile.com (also searches several $$ stock services)
Lost and Taken - Texture stock photos
Smithsonian Institution Open Access
New York Public Library Digital Collections (search box shows an “only public domain” option)
Library of Congress “free to use” collections