AfterLogic WebMail Lite PHP is a free AJAX webmail application. Users can receive, view, manage, compose, and send email through web interface (IMAP and SMTP supported). Source code included.
This plugin will add to the AJAX Webmail RoundCube the ability to tag mails like it is done in Thunderbird. He will follow the same semantic as Thunderbird to manage tag, so tags set using this plugins will be seen in Thunderbird and vice-versa.