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PBS is a great site for students to view many different videos on various curriculum topics. At our school the most popular viewed videos are from the science and history curriculum. I have also found some great math videos such as "fractals" that provide a real-life link to naturally occurring mathematics. All videos on this site are based on research and factual information that allow students to think critically about the topics they are studying.
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Discovery Education is a site that streams videos on curriculum topics. In my experience, these videos are more for teaching curriculum specific concepts and exploring relationships among concepts. You can search across the curriculum for any topic of your choice but you must have an account (our school system does).
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Brightstorm is an excellent digital video resource for math (algebra through calculus) and science (biology, physical science, and physics). The videos on this site showcase an instructor explaining a concept and working through two to three examples of how to apply the skills in a problem. The viewer has a choice to stop the video and play the examples in the order of their choice. This site is great for tutorials and review in math and science classes.
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SchoolTube is an online community of teachers and students who create their own videos. Teachers can upload lesson plans and recorded lessons to the site and share among the members. Students can view the videos on this site as an extension of what is covered in class and investigate critical thinking topics. There are searchable categories that span the educational curriculum and teachers and students can enter contests with their published videos.