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.
Rainloop - RainLoop is a web based email client.
Squirrelmail - SquirrelMail is a standards-based webmail package written in PHP.