Zimbra
From iA wiki
See also: Software | Groupware
Home Page: http://www.zimbra.com
From Freshmeat entry:
"A server and client technology for next-generation enterprise messaging and collaboration (email, group calendar, contacts, etc). It features a browser-based AJAX client with search, shared calendar, and mail that is integrated with contacts and calendar, and support for Outlook, iCal, Mail.app, contacts, and offline mode. Administrators benefit from online move, back-up, and recovery of individual or groups of mailboxes, compatibility with existing LDAP directories., native hierarchical storage management and clustering, Web services integration with existing enterprise applications, and integrated anti-spam and anti-virus."
Flash demo: http://www.zimbra.com/demos/zimbra_overview.html
Zimbra isn't the only group doing this (see http://freshmeat.net/browse/27/) but the fact that they offer a free OSS version that's smooth (on par with gmail) is pretty cool.

