Pixnio – A large collection of high resolution public domain images. Free for personal and commercial use, no attribution required.
Smithsonian Institution – Open Access – 2+ million free, downloadable public domain images from the Smithsonian's collections, no permission required.
The Met collection - Explore more than 406,000 hi-res images of public-domain works from the Metropolitan Museum of Art, all of which can be downloaded, shared, and remixed without restriction.
Public Domain Image Resources - Wikipedia-maintained page of public domain image resources, with many specialized collections. Note that not all of the sites listed contain content that is exclusively in the public domain.
Pixabay- Royalty-free images.
Unsplash - Freely usable images.