Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
ElectronMail - Unofficial desktop app for ProtonMail and Tutanota end-to-end encrypted email providers
OpenKeychain - OpenKeychain helps you communicate more privately and securely.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.