GnuPG is the GNU project's complete and free implementation of the OpenPGP standard as defined by RFC4880. 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.
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: 1.4.16 is the well known and portable standalone version, whereas 2.0.22 is the enhanced and somewhat harder to build version.
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.
Project Aegypten developed the S/MIME functionality in GnuPG 2.