The US National Archives and Records Administration (NARA) has put online over 50 million historical documents. For background on this project, see this Washington Post article.
This is simply amazing. As an example of what people can do with this data, see this sample search, which presents the following question:
Suppose you had always heard that your Uncle Quentin Smith had been in the Army and had been wounded during the Korean War. Now you want to learn more about what had happened to him. How might you go about finding out about this incident using AAD?
Bravo to whoever is responsible for this! (Link via How Appealing.)