This review by Theodoric Meyer and Peter Maass at ProPublica by is a must read for any American who wants to know which of their digital breadcrumbs are being picked up by the police and other government agencies:
The government isn’t allowed to wiretap American citizens without a warrant from a judge. But there are plenty of legal ways for law enforcement, from the local sheriff to the FBI to the Internal Revenue Service, to snoop on the digital trails you create every day. Authorities can often obtain your emails and texts by going to Google or AT&T with a simple subpoena that doesn’t require showing probable cause of a crime. And recent revelations about classified National Security Agency surveillance programs show that the government is regularly sweeping up data on Americans’ telephone calls and has the capability to access emails, files, online chats and other data — all under secret oversight by a special federal court.