HEY - Email at its best, new from Basecamp.
Roundcube - Web-based IMAP email client
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
InstAddr - Disposable email address application.
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.