By far the largest site of CC-licensed images is Flickr: The Commons. It claims to have over 220 million CC-licensed images, and the site allows you to search by the type of license you want. Use the Advanced Search feature to filter the results.
Everystockphoto is a photo search site which indexes and searches millions of freely licensed photos from many sources. It features a sophisticated advanced search feature which allows you to search by specific CC licenses or for public domain images.
WikiMedia Commons includes images and other media under CC licenses.
USA.gov has a portal to the immense image collections of US government agencies, most of which are in the public domain and are free of restrictions for non-commercial use except for attribution. There are some exceptions, however, so check the terms of each agency.
One exception is the image collection of the Smithsonian Institution. While not in the public domain, most of the images are available for non-commercial personal or educational use.
The Library of Congress has a massive catalog of digitized photographs, most of which, though not all, are in the public domain. (You can check the catalog records for this information. Click on the record for the collection you want, and then click on "Rights and restrictions.") However, the Library has compiled a helpful page of the most requested images which have no known restrictions on their use.
US Government Maps: The U.S. government also maintains a portal to maps created by federal agencies, states, and other sources. (Note: Maps not created by federal agencies may not be freely usable. Check the terms of service for non-federal agency sites.)
Note in particular the USGS National Map Viewer, which pulls together cartographic data from several federal agencies and permits you to customize the view, and display and print layers of geographic, topographic, political, demographic etc. data.
Other sources of maps:
Geographicus: 2000+ images of rare and antique maps donated by the antiquarian map dealer Geographicus, available under the Wiki Commons terms.
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