There are a lot of video sites out there now - so much stuff is being uploaded it's hard to find what you are looking for, even with a good search engine.
NEOK12 to the rescue. Not a video site, but an indexed repository of videos from a range of different servers, all vetted for use in education. The videos are streamed through their site so if your school blocks YouTube this might be a way around it.
Use these videos with TurningPoint Anywhere and you have a visual and interactive lesson to start a topic and an opportunity to check prior understanding, prediction and comprehension.