Roundcube - Web-based IMAP email client
HEY - Email at its best, new from Basecamp.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Rainloop - RainLoop is a web based email client.
InstAddr - Disposable email address application.
Squirrelmail - SquirrelMail is a standards-based webmail package written in PHP.