After countless hours of searching for powerpoint presentations related to Earth and Physical Sciences last year, I've come across a site that proves to be hopeful as a powerpoint search engine. The PowerPoint-Search.com site searches powerpoints on the web by topic and provides an array of files specific to the query without having to weed through the results looking for specific file types.
Check it out.
Most authors on the web are the sharing type. Still, it's appropriate to give credit to an author when you use their work in whole or part.