Encyclomedia is a free site that contains a huge variety of videos on history, science, nature, people, travel, entertainment and much more.
They have designed it as an online video encyclopedia with descriptions and information about each video. You can browse by category or search for different videos.
There are a bunch of popup/linked ads, but they have to keep the site running somehow.
Take a look and see if there are any videos you can use in your classroom.