GnuPG is a complete and free implementation of the OpenPGP standard encryption. 

Installing GPG

GPG is available in Windows and many distributions of Linux, including a graphical user interface (GUI) that is easy to use. However, I prefer using the command line tools instead.

Red Hat / CentOS

sudo yum install gnupg

Ubuntu / Debian

sudo apt-get install gnupg

Note: Use gpg2 (version 2.x)