While currently still in Beta mode, the Digital Public Library of America brings together the riches of America’s libraries, archives, and museums, and makes them freely available to the world. It strives to contain the full breadth of human expression, from the written word, to works of art and culture, to records of America’s heritage, to the efforts and data of science.
The developers have released an API that allows developers to utilize the massive amount of information in new an innovative ways. I will be sifting at this site for a while!