A modern browser-based multilingual IMAP client
http://www.roundcube.net/

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, message searching and spell checking.

Roundcube Webmail is written in PHP and requires the MySQL database.
The user interface is fully skinnable using XHTML and CSS 2.