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.
Horde Groupware Webmail Edition is an enterprise ready browser-based communication suite. Users can read, send, and organize email messages and manage and share calendars, contacts, tasks, and notes with the standards compliant components from the Horde Project. It bundles the separately available applications IMP, Ingo, Kronolith, Turba, Nag, and Mnemo. It can be extended with any of the released Horde applications or the Horde modules that are still in development, like a file manager, a bookmark manager, a forum, or a wiki.
AfterLogic WebMail Lite PHP is a Webmail script with a modern AJAX interface. It supports POP3/IMAP accounts, SMTP, and SSL (including Gmail). Users can check and send mail and use an address book. Skins, a 3-pane view, non-English languages (including right-to-left Arabic and Hebrew), UTF-8 and other charsets, IMAP folders, and view and compose HTML mail. MS SQL/MySQL backends are supported, as well as multiple mail accounts and domains, Web-based administration and installation, and cPanel integration.