Roundcube - Web-based IMAP email client
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Geary - Geary is an email application built for GNOME 3.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.