Entire website, everything is CC0 or practically so (same rights):
- https://www.pexels.com/
- https://pixabay.com/
- https://unsplash.com/license
- https://www.sharetextures.com/about-2/
- https://openclipart.org/share
- https://www.needpix.com/about
Specific sections of website contain CC0 work:
- The Metropolitan Museum of Art's "Open Access" artwork: https://www.metmuseum.org/art/collection/search?showOnly=withImage%7CopenAccess&pageSize=0&sortBy=Relevance&sortOrder=asc&searchField=All
- Raw Pixel's Public Domain section: https://www.rawpixel.com/public-domain
- Free Sound . org filtered by CC0: https://freesound.org/search/?g=1&f=%20license:%22Creative+Commons+0%22&s=None&cluster_id=