ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Tutanota - Get your encrypted mailbox for free.
Roundcube - Web-based IMAP email client
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.