PowerPoint Home Page at Microsoft.
Microsoft Support Knowledge Base: Search for articles from Microsoft's Product Knowledge Base.
The PowerPoint FAQ: The most comprehensive PowerPoint FAQ. It is maintained by Microsoft MVP for Microsoft Office PowerPoint, Steve Rindsberg.
PPTools: A collection of PowerPoint tools that make you more productive with PowerPoint.
Indezine - A PowerPoint E-zine: An Online PowerPoint Magazine authored by Geetesh Bajaj.
OfficeTips: A site with a lot of useful VBA code snippets by Shyam for Microsoft Office PowerPoint.
Tutorials from PowerPoint MVP Echo Swinford.
PowerPoint Backgrounds has some first-rate tutorials on linking, using images in PowerPoint and more.
Excel MVP Tushar Mehta also has a PowerPoint section on his site with some useful information.
PowerPoint Workbench: Glen Millar's PowerPoint site.
OnPPT: Get advice on PowerPoint at OnPPT.
Presentation Magic from the PowerPoint Magician - PowerPoint MVP Glenna Shaw - focuses on accessibility issues.
PowerPoint Heaven: A good collection of animation samples.