Roundcube - Web-based IMAP email client
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
HEY - Email at its best, new from Basecamp.