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 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
2.0.7 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.
The GNU Privacy Guard - GnuPG.org
GnuPG 
Linear Mode
