Roundcube - Web-based IMAP email client
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Squirrelmail - SquirrelMail is a standards-based webmail package written in PHP.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Atmail - Save time and scale faster with accessible, secure, and reliable cloud email services. A proven multitenant email solution for telcos and service providers.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.