Google Cloud Connect for Microsoft Office easily adds collaborative tools to Microsoft Office.
Update: Google have withdrawn this product (http://googleappsupdates.blogspot.com.au/2013/03/cloud-connect-deprecation.html)
I am not a fan of Microsoft Office – it is expensive, bloated and the file formats are propriety; however many of my clients are using it (despite my recommendations to use OpenOffice or Google Apps).
One of the limitations of Microsoft Office is that it can be difficult for people to collaborate on a document or spreadsheet. You can try emailing each other the latest revisions but merging the changes can be a nightmare, and keeping the document on a common network path is a recipe for disaster as revisions get copied over the top of each other.
A way around this is to use Google Cloud Connect for Microsoft Office. This installs a toolbar into the Office suite which then allows authors to share their documents using Google Apps. The document remains in its native MS format but Cloud Connect with take care of managing the revisions.
One of my clients is using this to manage their staff roster spreadsheet that is being updated from two physical locations simultaneously. Previously they had been using email with much confusion; now everybody knows where they are supposed to be.