GPGMail - GPGMail is an open source plugin for Apple Mail.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
ProtonMail Extension - Unofficial webextension for ProtonMail