Blinkx describes itself as "the world’s largest and most advanced video search engine". It uses "a unique combination of patented conceptual search, speech recognition and video analysis software to efficiently, automatically and accurately find and qualify online video".
I have tried searching for appropriate videos for some of my teaching on many search engines but this really did find more than other search engines. If you are looking for a video and are not finding it easily, try Blinkx. You can search by category or use your own search terms.