The following sites offer many extra add-ins and useful utilities.
www.webaim.org/techniques/powerpoint/
If you are interested in making Web-posted PowerPoint presentations more accessible to people with disabilities, this is a must-read. It contains a link to an Office Accessibility Wizard, which is an add-in that converts PowerPoint presentations to a format that is usable by screen-reading programs for the visually impaired and is easily navigable.
Here's the Web site of a PowerPoint MVP who also has created some great add-ins and converters that you'll find included on the CD at the back of this book. He also runs www.pptfaq.com where you'll find PowerPoint FAQs.
Makers of the PowerPlugs for PowerPoint product, a set of add-ins that provide additional animation, 3-D, transition, charting, and graphics capabilities within PowerPoint.
Source of dozens of wonderful add-ins for PowerPoint, many of which have been referenced throughout this book. This site is run by Shyam Pillai, another of the PowerPoint MVPs and an expert in extending PowerPoint with Visual Basic for Applications.