July 31, 2013: XKeyscore revealed to the public (Snowden document)

It was one year ago that the Guardian revealed the XKeyscore program which was used to create an enormous database of emails, online chats, and browsing histories.  The information that was collected was made freely available to NSA analysts, who could browse it at will, without having to appeal for a warrant. The significance of that revelation was contained in the fact that this information was gathered on American citizens, who are, by right, "secure from all unreasonable searches, and seizures of his person, his houses, his papers, and all his possessions."  (cite)

The NSA boasted in its own training materials that XKeyscore is its "widest-reaching system for developing intelligence from the internet" and covers "nearly everything a typical user does on the internet."

The Guardian UK, July 31, 2013:
http://www.theguardian.com/world/2013/jul/31/nsa-top-secret-program-online-data 

XKeyscore was one of the most important top secret programs that Edward Snowden revealed to the American public and the program he was referring to when he said, "I, sitting at my desk, could wiretap anyone, from you or your accountant, to a federal judge or even the president, if I had a personal email."