GnuPG is the GNU project's complete and free implementation of the OpenPGP standard as defined by RFC2440 . GnuPG allows to encrypt and sign your data and communication, features a versatile key managment system as well as access modules for all kind 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 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: 1.4.7 is the well known and portable standalone version, whereas 2.0.4 is the enhanced and somewhat harder to build version.
Project Gpg4win provides a Windows version of GnuPG. It is nicely integrated into a installer and features several frontends as well as (German) manuals.
Project Aegypten developed the S/MIME functionality in GnuPG 2.
you can find out more information about GPG at: www.gnupg.org