Zimbra is open source server and client software for messaging and collaboration - email, group calendaring, contacts, and web document management and authoring. The Zimbra server is available for Linux, Mac OS X, appliances, and virtualization platforms.
Zimbra will announce a new offline client application, Zimbra Desktop, later this week. It will allow Zimbra users to access and use Zimbra’s email and other office applications, in the browser, when offline.