Tor: An anonymous Internet communication system
Stands for "The Onion Routing"
Tor is a toolset for a wide range of organizations and people that want to improve their safety and security on the Internet. Using Tor can help you anonymize web browsing and publishing, instant messaging, IRC, SSH, and more. Tor also provides a platform on which software developers can build new applications with built-in anonymity, safety, and privacy features.
Installing Tor with privoxy to surf anonymously
The CCC Anonymizer (tor and jap server)
related: onion-router.net, Wikipedia:Onion_Routing, [Good Privacy Test Sites]