Roundcube is a modern webmail client that will install easily on a standard LAMP (Linux, Apache, MySQL, and PHP) stack. It features a drag-and-drop interface that generally feels modern and fast, and comes with a slew of features: canned responses, spell checking, translation into over 70 languages, a templating system, tight address book integration, and many more. It also features a pluggable API for creating extensions.It comes with a comprehensive search tool, and a number of features on the roadmap, from calendaring to a mobile UI to conversation view, sound promising, but at the moment these missing features do hold it back a bit compared to some other options.Roundcube is available as open source under the GPLv3.