Roundcube Webmail is a browser-based, multilingual IMAP client with an application-like user interface. It provides the full functionality you expect from an email client, including MIME support, address book, and folder manipulation. High extensibility is granted by the plugin API and the user interface is fully skinnable using XHTML and CSS 2.
AfterLogic WebMail Lite PHP is a Webmail script with a modern AJAX interface. It supports IMAP accounts, SMTP, and SSL (including Gmail). Users can check and send mail and use an address book. It provides a slick skin, a three-pane view, non-English language support, UTF-8 support, IMAP folders, and viewing and composing HTML mail. A MySQL backend is supported, with Web-based administration and installation and cPanel/Plesk/DirectAdmin integration.
Insight Server is a Linux-based email server utilizing open source components to provide a highly reliable, scalable, and cost effective email solution for customers of all sizes. It's designed to facilitate complete messaging and collaboration capabilities within a company. It supports all standard protocols, and provides backup and recovery tools, server redundancy, migration tools, and resource management.