AtMail is an open source webmail client written in PHP. We aim to provide a elegant Ajax webmail client for existing IMAP mailservers, with less bloat and a focus on an intuitive, simple user interface.
The open source version of AtMail provides users with a lightweight, yet powerful webmail client. The software can be installed on a variety of platforms with ease and without the hassles that most webmail platforms impart.
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 a MySQL or Postgres database. The user interface is fully skinnable using XHTML and CSS 2.
SquirrelMail is a standards-based webmail package written in PHP. 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. It has very few requirements and is very easy to configure and install. SquirrelMail has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation.