SecureDrop (originally DeadDrop) is a web application developed by the late Aaron Swartz, posthumously released in 2013, designed to allow for untraceable pseudonymous exchange of messages, particularly between news organizations and their sources. Somebody can exchange a whole series of messages and become a trusted source under a system-assigned codename without their real-world identity being exposed, or even findable should authorities demand the journalist reveal it. The Tor network and PGP encryption are used.
The first high-profile user of this service is The New Yorker magazine, which has set up its Strongbox service using DeadDrop/SecureDrop. POGO (Project On Government Oversight) has also installed a version of it, as has the Washington Post.