Roundcube - Web-based IMAP email client
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Rainloop - RainLoop is a web based email client.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Squirrelmail - SquirrelMail is a standards-based webmail package written in PHP.
HEY - Email at its best, new from Basecamp.