The top few sites I found include Free PowerPoint Templates, Indesign, Sonia Coleman’s Digital Studio, Brainy Betty, TemplatesWise, Presentation Helper, Powerbacks, Powered Templates, etc. Each offered a large number of choices organized in specific categories. One site offered 600 free templates and banners like that trying to attract you to the site. These are the top sites in the general category. They offer a wide variety and are not geared to any specific category.
These sites are catering to specialized themes. The Free PowerPoint Templates had the templates organized in Abstract, Medicine, Nature, Patterns, Shapes, Textures, General, Maps & Globes and color.
Sonia Cloeman's offerings were organized in sets. They had much more to offer in their premium sets.
Brainy Betty offers sets organized by their own criteria. But what distinguishes them are the additional tools they offer for making professional PowerPoint presentations. These include Flash tools. Premium offers are also available.
Templateswise has their offers arranged in abstract, business, finance, general, nature and travel themes.
You might wonder how they can give away these so freely. Many have a business model offering you the free samples and getting you hooked. They have premium items that need to be bought if you want to use those options. Others had ads displayed on their sites. Most often, Adsense or some other ad service providers’ ad program. So these sites attract traffic and then tried to earn money from the ads.
Whatever business model used, they offered a lot of free choices. Some sites offered additional resources. Some of these offered training on PowerPoint. Some had additional resources of articles to read and make your presentations even more effective.