The first piece of PublicKeyCryptography for the masses. PhilZimmerman wrote this and released it to be out before a law against encryption passed in order to have it be grandfathered.
While public encryption has gone from this to SecureSocketsLayer, SecureShell and into almost every browser, this was where the legal issues first came out.
http://www.pgp.com/ -- PGP Corporation http://www.pgpi.org/ -- The International PGP Home Page http://www.gnupg.org/ -- GNU Privacy Guard, a GPLed implementation
PGP incorporated a WebOfTrust in its design. PGP users were encouraged to digitally sign the public keys of other PGP users they trusted. The number of trusted signatures was used to determine the trust level of unfamiliar public keys.