Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
HEY - Email at its best, new from Basecamp.
Roundcube - Web-based IMAP email client
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
InstAddr - Disposable email address application.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.