Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Seahorse - Front end for GnuPG.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
OpenKeychain - OpenKeychain helps you communicate more privately and securely.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.