These are primarily resources that have public domain images. I've tried to leave out resources that provide free access, but not necessarily public domain materials. As always, please do your own research before reusing content you find online.
See the Finding Creative Commons content page for images that are licensed for reuse. Also, be sure to check out the Federal Government Websites page because there are several government image databases.
This work is licensed under a Creative Commons Attribution-NonCommercial 2.0 Generic License.