monline

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.

Apart from the above promo, I've used it and it rocks. Setup is a piece of cake and despite of being only 0.1 RC1 (man ... I love OSS versioning ) it works pretty well. I've found only a small issue, but one can live with that.