SquirrelMail is a standards-based webmail package written in PHP4. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages render in pure HTML 4.0 (with no JavaScript required) for maximum compatibility across browsers.

Website -> http://squirrelmail.org/

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.

Website -> http://roundcube.net/