The DarkMarket Sting
This one isn't a single event, as much as it is an incredible sting operation. From roughly 2006 to 2008, a large online community, called DarkMarket, existed to facilitate the sales of stolen information and other nefarious online activities. Thankfully, it was run by the FBI.
For two years, the webmaster “Master Splynter" was an undercover agent operating out of the Pittsburgh branch of the FBI's cybercrime division. Despite several close calls, the cover mostly held, and led the arrest of 56 cyber criminals around the world, with coordinated raids in the United States, the United Kingdom, Turkey and Germany. The FBI estimates that nearly $70 million in damages was prevented by the sting.
Perhaps the most remarkable part is the end though. As the sting came to a close and the arrests rolled out in force, “Master Splynter" posted a simple goodbye on the site, claiming that increased law enforcement attention meant that he had to disappear into the night. It took a full month for the cover to be blown, when FBI documents noted the success of the operation and news spread.