Free/Libre/Open Source Licensed Materials
The main website where the Creative Commons is organized.
Flickr Creative Commons allows one to register and upload images. Users may search Flickr Creative Commons using keywords or browsing tags. One may browse by a particular Creative Commons license.
OpenPhoto includes Creative Commons, as well as Public Domain licensed photos. Selection is still limited.
yotophoto offers a single search interface for finding images available with generous use licences.