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