ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Riseup - Riseup provides online communication tools for people and groups working on liberatory social change. We are a project to create democratic alternatives and practice self-determination by controlling our own secure means of communications.
Tutanota - Get your encrypted mailbox for free.
GPGMail - GPGMail is an open source plugin for Apple Mail.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
Posteo - Web- and IMAP-based email service. 2GB cost 1€/month.