GPG

GNU Privacy Guard (Gnu P G or GPG) is a GPL Licensed alternative to the PGP suite of cryptographic software. Gnu P G is compliant with RFC 4880, which is the current IETF standards track specification of Open P G P. Current versions of PGP (and Veridis' Filecrypt) are interoperable with Gnu P G and other Open P G P-compliant systems. http://en.wikipedia.org/wiki/GNU_Privacy_Guard

Feb'2015: I think of GPG as a glorious experiment that has run its course. The journalists who depend on it struggle with it and often mess up (“I send you the private key to communicate privately, right?”), the activists who use it do so relatively sparingly (“wait, this thing wants my finger print?”), and no other sane person is willing to use it by default. Even the projects that attempt to use it as a dependency struggle.