Roundcube - Web-based IMAP email client
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
mailcow - An open source mailserver suite.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.