Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license.

NASA content - images, audio, video, and computer files used in the rendition of 3-dimensional models, such as texture maps and polygon data in any format - generally are not copyrighted.

Limit your search to "Open Access Available."

More than 180,000 of the items in NYPL's Digital Collections are in the public domain

"The Getty makes available, without charge, all available digital images to which the Getty holds the rights or that are in the public domain to be used for any purpose. No permission is required. "

"All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required."

"Whether you’re a filmmaker, musician, designer, student, or just a history buff, the Pond5 Public Domain Project is making copyright-free media available for you."

As the name suggests, all the historical works found on this site are in the public domain, at least somewhere in the world.

Images on the Rijksmuseum website are fully searchable and downloadable. Wherever the image is in the public domain, this information is listed in "acquisition and rights."

Photos in the Image Gallery are copyright-free, public domain, images unless otherwise indicated.

"Wikimedia Commons only accepts free content... that [is] not subject to copyright restrictions which would prevent them being used by anyone, anytime, for any purpose."