The brand new Creative Commons search engine is something the Internet has waited for:
The Creative Commons search engine gives you access to over nine million images drawn from 500px, Flickr, the Metropolitan Museum of Art, the New York Public Library and the Rijksmuseum. You can search through all or any combination of these collections. You can also constrain your search to titles, creators, tags or any combination of the three. Finally, you can limit your search to images that you can modify, adapt or build upon as you see fit, or that are free to use for commercial purposes.