Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
OpenKeychain - OpenKeychain helps you communicate more privately and securely.
ProtonMail Extension - Unofficial webextension for ProtonMail
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
GPGMail - GPGMail is an open source plugin for Apple Mail.