Ookaboo: free pictures of everything on earth

Collage of Lakota people from various public domain sources. 

"Ookaboo is a collection of free pictures, indexed by precise terms from the semantic web. All pictures on Ookaboo are in the public domain or are under Creative Commons -- that means that you can use our pictures for your web site, classwork, or other creative projects! All data
Images on Ookaboo are indexed by terms from the semantic web, the web of linked data. Although you're free to find images through the human interface, automated systems can quickly find and use images through the semantic API.
Ookaboo has two goals: (i) to dramatically improve the state of the art in image search for both humans and machines, and (ii) to construct a knowledge base about the world that people live in that can be used to help information systems better understand us..." Ookaboo