Bynari's InsightConnector is a MicrosoftOutlook plug-in that allows full Outlook GroupWare capabilities without the need for a MicrosoftExchange server. With Insight Connector installed, together with Insight Server, Outlook users can perform such tasks as Calendars, Folders, and Contacts sharing, Appointments/Tasks scheduling, and other collaborative tasks.
http://bynari.net/index.php?id=7
Note; this connector uses a hack on the server to store calendar objects in an IMAP folder, so it's all or nothing with this - you can only realistically use Outlook to access a Bynari store.
One page links to InsightConnector: