Office Tab ($25, 30-day free trial) solves one of Microsoft Office's shortcomings: How to handle multiple documents open in different windows. Unlike Web browsers, Office doesn't use tabs, so you can't see at a glance all the documents you're working on, and it's not easy to switch between them. Office Tab solves the problem by putting tabs on Office, Excel, and PowerPoint.
Every time you create or open a new document, that document runs in its own tab. To switch between tabs, simply click on the one you want to use. You can also use keyboard shortcuts to switch as well.
You can also customize how the tabs work, for example, by determining where to place them, what shortcuts to use, and whether to hide the Tabs bar when only one tab is open.
At $25, Office Tab isn't overly expensive. If you often find yourself fumbling around when switching between open documents, it may well be worth it for you.