Citadel is a turnkey open-source solution for email and collaboration. One simple installation delivers a multitude of powerful features, including: email, calendaring/scheduling, address books, bulletin boards, mailing list server, instant messaging, multiple domain support, modern AJAX-style web interface
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 a MySQL or Postgres database. The user interface is fully skinnable using XHTML and CSS 2.