The Quick Access toolbar is a powerful feature in the MS Office applicaitons. Excel, Word and PowerPoint have this feature. While this might look like Microsoft has included this feature to pacify those who were used to the older toolbar Graphic User Interface (GUI) and who are now facing a new GUI in the form of the Ribbon (Fluent - as called by MS) interface, it is actually a boon to those who might want to use a couple of commands again and again very frequently during a session in the respective application. And the added advantage here is that it is customizable - you can include any command there and use it conveniently.
Apart from the standard commands made available directly, there is also a provision for adding any command when the user clicks the "More commands" option. You can also include an icon here that represents a macro you have built and have stored it in a seperate file (which usually lies hidden from the user).
There is also the provision for placing this Quick Access toolbar at the bottom of the Ribbon interface for those who are finding it difficult to leave behind the traditional way of working in these applications.
A couple of commands that might be of great use to place in the Quick Access toolbar (apart from Save, Undo and Redo) are:
Print Preview - while this is still accessible from the file menu (office button in 2007), an immediate preview of the Word document to see how it will print is a great time saver.
Email - You can attache the current document/workbook/presentation immediately to a mail (make sure to save it first to give it an appropriate name (failing which you file will contain wierd names like Document1.docx or Book1.xlsx or presentation1.pptx). It is also essential that you have a mailing application profile (Outlook) active under your login name in the system you are currently working. It is another great time saver.