GPGMail - GPGMail is an open source plugin for Apple Mail.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Tutanota - Get your encrypted mailbox for free.
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).