http://www.youtube.com/schools allows you access to thousands of free, high-quality educational videos on YouTube in a controlled environment.
You can customise the content available in your school. All schools get access to all of the YouTube EDU content, but teachers and administrators can also create playlists of videos that are viewable only within their school’s network.
YouTube.com/Teachers has hundreds of playlists of videos that align with common educational standards, organised by subject and class. These playlists were created by teachers for teachers, so you can spend more time teaching and less time searching.