The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
ElectronMail - Unofficial desktop app for ProtonMail and Tutanota end-to-end encrypted email providers
Geary - Geary is an email application built for GNOME 3.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.