GnuPG stands for GNU Privacy Guard, and is a widely used Free Software for mail and data encryption and signing.
GnuPG, is a modular crypto engine allowing you to encrypt and sign your data and communications; it features a versatile key management system, along with access modules for all kinds of public key directories. It integrates with other applications. A number of frontend applications and libraries are available. The main command line interfaces are gpg and gpgsm.
GnuPG is a complete and Free Software implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
It also implements the Cryptographic Message Standard (CMS), needed for S/MIME emails and provides supports for Secure Shell (ssh).
Since its introduction in 1997, GnuPG is Free Software (meaning that it respects your freedom). (Some call this "Open Source".) GnuPG can be used, modified and distributed under the terms of the GNU General Public License.
See Download page for both source code and binary releases. And there is also a ftp mirror list.
Gpg4win is a Windows version of GnuPG featuring a context menu tool, a crypto manager, and an Outlook plugin to send and receive standard OpenPGP/MIME mails.