A Search Engine for Free to Use Photos

[GizmoBird]

Can even search by color.

Information about licensing from the site:
"About the images
Usage conditions
While some of the images are in the Public Domain, most are released under Creative Commons, GNU FDL or similar licenses. These licenses typically have some minor conditions of use (such as requiring that credit be given to the photographer). Yotophoto offers resources and links to help users understand and comply with the license conditions (because we hate legalese as much as you do). "
They also have information about the requirements of each license. This is a fantastic resource.

http://yotophoto.com/