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.
Rainloop - RainLoop is a web based email client.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.