"Webmail.us LLC, a business-class e-mail-hosting specialist and the mail division of Rackspace Managed Hosting, announced the launch of Outlook-Sync, a service that allows business email users to synchronize web-based shared calendars, contacts, and task lists with Microsoft Outlook. "Based on the Funambol open source software project, Outlook-Sync leverages SyncML wireless technology to share data between the web and Outlook. Outlook-Sync, which is installed as a plug-in to Microsoft Outlook, can increase productivity by allowing business email users to access their shared calendars, contacts, and task lists from Outlook or webmail without having to enter data into both locations."
I'm not promoting Webmail.us, but I like to see companies that have taken my advice (using Funambol and marketing Sync to business email users). It a crowded marketplace, being different means sales (if marketed and branded correctly).
There's even a decent HOWTO available.