Concurrent Excel Workbook connections over the internet?

Essentially I have an Excel file that is going to need to be worked on concurrently for a prolonged period of time. In the past I would simply 'Share the Workbook' and this would allow users on the network to view/change the file at the same time as other users on the network but in this particular instance everyone is disconnected from a centr开发者_运维问答al network and there is no mechanism available to place them on the same network. Does anyone know of a service out there that will allow all parties to edit this document in a central location concurrently?

A MOSS box came to mind but that seems like overkill for a single document. Thanks.

You could use SharePoint Foundation 2010 (free if you have Windows Server 2008 - so maybe a better fit than paying for MOSS) and import the Excel file as a list, and then sync the list with Excel.

Or, Google docs has a spreadsheet program - I think it allows importation of Excel spreadsheets.