GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
GPGMail - GPGMail is an open source plugin for Apple Mail.
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey