I have been playing with the co-authoring function in Office 2010, SharePoint 2010 and the Office Web Apps. Co-authoring allows simultaneous editing of the same document by multiple users. It took me some time to figure out which apps actually support this functionality, because not all of them do.
The following site gives a good overview: http://office.microsoft.com/en-us/sharepoint-server-help/document-collaboration-and-co-authoring-HA101812148.aspx
The table below (taken from the article) is a good summary:
Application
Server Requirements
Common Scenarios
Word 2010
SharePoint Foundation 2010
Any document including proposals, plans, vision statements, minutes, newsletters, and reports
PowerPoint 2010
SharePoint Foundation 2010
Any presentation including training, conferences, post-mortems, product overviews, handbooks, and project status reports
Excel Web App
SharePoint Foundation 2010
and Office Web Apps
Any spreadsheet, including team financial modeling, business-to business product line update on a web page, and real-time trading spreadsheet trackers
OneNote 2010
SharePoint Foundation 2010
Any notebook, including recurring meeting minutes, project brainstorming (“group-think”), shared research and reference material, and shared training courses
OneNote Web App
SharePoint Foundation 2010 and Office Web Apps