Tor (The Onion Router) is a free software implementation of second-generation onion routing – a system enabling its users to communicate anonymously on the Internet. Originally sponsored by the US Naval Research Laboratory, Tor became an Electronic Frontier Foundation (EFF) project in late 2004. The EFF supported Tor financially until November 2005, and continues to provide web hosting for the project.

Like all current low latency anonymity networks, Tor is vulnerable to traffic analysis from observers who can watch both ends of a user's connection.