Shutterstock - they actually have a free photo section; some images are for editorial use only, which means they cannot be used for commercial purposes, so make sure you check the licensing for any free image you intend to use commercially
Pixabay - all images uploaded here before 09 Jan 2019 have a public domain license and those images can be used without a license and without attribution; the site/content was purchased by Canva on 09 Jan 2019 and started issuing their own licenses, but they could not legally revoke the public domain status of images uploaded prior to 09 Jan 2019, so those images are still in the public domain
Gratisography - scroll past the ads
picjumbo - X out [top-right] of the overlay if you're not interested in getting images in your email; the freebies are below the overlay
Nappy - dedicated to black and brown people
Wikimedia Commons - make sure you choose images with the Creative Commons Zero, Public Domain Dedication license, and then you can use it without restriction and without attribution