RoundCube Webmail Project
Cool looking email client with lots of ajax. I’m definitely going to give this a look.
RoundCube Webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation and message filters. RoundCube Webmail is written in PHP and requires the MySQL database. The user interface is fully skinnable using XHTML and CSS 2.