yotophotoYotoPhoto is an image search engine with a difference. It allows you to search for free-to-use stock photographs and images. These images are either in the Public Domain or released under generous Creative Commons, GNU FDL or similar licenses.

In short these are images you can use without paying anything. However an attribution may be required in some cases. They also provide a link to the type of license the image is under for convenience.

They index various sources of free imagery on the web including Flickr, Wikipedia, Stock.Xchng, Morguefile, Pixelperfect Digital and OpenPhoto. Only photographs are indexed at this time. Other image types like maps, graphics, illustrations, 3D renderings, charts and the like are not indexed currently.

You can search by tags, keywords or by color. You can include stop words and specify image orientation. You can also combine keyword search with color attribute. So if you want to find an exit sign in green you visit:
http://yotophoto.com/images/exit/1?color=00ff00

Overall my impression was positive.