Tor
Initially known as "The Onion Router", Tor is an Internet based overlay network of Tor Relays which work together in order to provide web anonymity for Tor users. In essence, the Tor network will use several layers of encryption in order to obfuscate the origin of the web traffic, hence keeping the source IP address of the Tor user secret. Tor also supports "Tor Hidden Services", which are websites only accessible from the Tor network. Tor hidden services use the Tor network to keep the origin of the website secret, hence making it difficult for law enforcement/government to shut illegal/censored websites down.
