Office 365 is a fantastic online collaboration system. Based on Microsoft's flagship e-mail product, Exchange 2010, it delivers enterprise grade e-mail and calendaring functionality for a fraction of what you might pay to have the same system installed on site. Also included in some of the plans is the use of a SharePoint 2010 server for document management and many other workflow features.
You pay per user per month and there are no lock in contracts.
Just in case you were looking for more reasons to move, Micorsoft have just reduced their prices by between 16-20%! What are you waiting for?