OfficeTab Adds Tabs to Microsoft Office:

Windows only: Microsoft Office plug-in OfficeTab adds tabbed document support to Excel, Word, and PowerPoint—one can only wonder why Microsoft doesn't include this feature by default.

Once you've installed OfficeTab, you can head into the settings panel, where you can enable or disable tabbed support in each application, move the tabs to the side or bottom, and change the appearance of the tabs with a number of styles to choose from. You can right-click the tab bar to add a new document tab, and each time you open a document it will be added to a new tab automatically.

OfficeTab is a free download for Windows only, and while it was created by a Chinese programmer, the download itself contains an English version. Readers should note that on our test machine, at least, we got a Visual Studio setup prompt every time we opened an Office application after setting up this plug-in—but your mileage may vary.