Mobento is one of those places. What makes Mobento different from most educational video hosts is that you can search for spoken words within a video. Mobento not only tells you which videos have specific words spoken in them, it will also show you a timeline of where in the video your specified words are spoken.
Applications for Education
Mobento's collection of videos isn't terribly large right now, but it has potential to be a great service for finding educational video clips that address the specific terms you're searching for.