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.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
GPG Suite - Use GPG Suite to encrypt, decrypt, sign and verify files or messages.