Microsoft Office: 40 of the Best Add-ins for PowerPoint

Check out these Add-ins for PowerPoint

http://www.powerusersoftwares.com/#!40-of-the-best-addins-plugins-and-apps-for-Microsoft-PowerPoint-free-or-not/biauv/56b62cf80cf26832893d6eb2

Now with the updated link.

Add-ins, similar to Google's Apps for Education Add-ons, add more features to Microsoft Office products. These are usually developed by other parties rather than by Microsoft.