Flickr has just made it easier for you to search photos that do not have copyright restrictions (that are known) on Flickr. The site is called The Commons.
If you go to http://www.flickr.com/commons and type in a keyword in the search box, you will find a collection of photos that do not have copyright restrictions. Some of the participating institutions for this project are The Library of Congress, Smithsonian Institution, George Eastman House, just to name a few. To learn more about The Commons or to try it out, visit the link below: