Office 2010 and SharePoint 2010 allow you to edit documents at the same time. Just select 'Edit in Microsoft [Word, Excel, etc]' in the context menu of the document that you want to edit and you will be collaborating with any other users that are using that document.

Check out this video that shows you how it works...

http://www.youtube.com/watch?v=hwzs-p_ym0w