Documentary Heaven - Watch Free Documentaries

Documentary Heaven is a free site that has organized more than 1600 documentary films found across the Internet. Through Documentary Heaven can find documentaries covering all kinds of topics in science, history, politics, business, and many more categories. The videos are sourced from a variety of services including, but not limited to, YouTube.

Applications for Education
I'm sure that you could find the videos on Documentary Heaven by searching the web yourself, but Documentary Heaven makes it easier to find good content. One word of caution though, Documentary Heaven does have some videos in the "lifestyle" category that might not be appropriate for K-12 use so you might not want to have students search the site on their own.