Wikipedia
Roundcube is a web-based IMAP email client. Roundcube's most prominent feature is the pervasive use of Ajax technology. After about two years of development, the first stable release of Roundcube was announced in early 2008.
Roundcube is written in PHP and can be employed in conjunction with a LAMP "stack", or any other operating systems that support PHP are supported as well. The web server needs access to the IMAP server hosting the email and to an SMTP server to be able to send mails.
Roundcube is free and open-source software subject to the terms of the GNU General Public License (GPL) with exceptions for skins and plugins.
On May 3, 2015, Roundcube announced that they plan a complete rewrite of Roundcube called Roundcube Next. A crowd funding was set up to finance the project. The goal of $80,000 was reached on June 24. The final stand was $103,541. Roundcube Next will also include additional features like calendar, chat and file management. This will be implemented using WebRTC and connectors from popular services like Dropbox and OwnCloud.