As a teacher, I use Microsoft Office applications for various purposes. If I can use shortcuts to save me a few seconds here and there, they add up. Microsoft add-ins are a great way to save time.
The add-ins plugs into Office and provide additional features not found in the default installation. You usually access them via the added Ribbon tab. There are two add-ins I use on a regular basis.
This add-in is free for home and student users and works with Excel 2000 (!) and above.
The second add-in is the Office Tabs. This adds tabbed editing and browsing of the open documents (workbooks and presentations). One downside to Office 2013 is that it does not supported tabbed view. That means when you open multiple workbooks in Excel, for example, they each show up in a different taskbar button at the bottom of the screen. Windows sometimes consolidates all the buttons as one button so you have to manually hover over the consolidated button and find the workbook you need to switch over. Install this add-in and managing multiple open workbooks because a breeze. Other features include saving all open files with one click, renaming files, and closing documents. Take a look at the web site for more features.