Do you use Microsoft Office 2016 for Mac? If you do, you'll want to get the latest update -- version 15.21.1 -- because it adds not only a lot of fixes and enhancements, but the Add-In Store for Word and Excel as well .
To get the update, just open any Office app -- Word, Excel, PowerPoint, Outlook or OneNote -- and then choose "Check for Updates" from the Help menu. The update takes a while as all of the apps are receiving attention, but once it's done, you can check out the products in the Add-In Store.
Many of the Add-ins, like the one from Wikipedia that allows research to be done from right in Word, are free. We didn't bother to look at what the rest of update 15.21.1 provides, but most Microsoft updates provide bug fixes and security patches, so it's a good idea to update.