GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP). GnuPG allows to encrypt and sign your data and communication, features a versatile key management system as well as access modules for all kinds of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications. A wealth of frontend applications and libraries are available. Version 2 of GnuPG also provides support for S/MIME and Secure Shell (ssh).
GnuPG is Free Software (meaning that it respects your freedom). It can be freely used, modified and distributed under the terms of the GNU General Public License .
GnuPG comes in two flavours: 1.4.18 is the well known and portable standalone version, whereas 2.0.26 is the enhanced and modern version and suggested for most users.
Project Gpg4win provides a Windows version of GnuPG. It is nicely integrated into an installer and features several frontends as well as English and German manuals.
Project GPGTools provides a Mac OS X version of GnuPG. It is nicely integrated into an installer and features all required tools.