WatchKnowLearn is a fabulous resource for students and teachers.  The site offers educational videos for many subject areas.  The vision is stated here:

To provide a world-class, online domain on which educators can store, categorize, and rate the best, K – 12 educational videos on the Internet today. And to make this service FREE so teachers, parents and students everywhere may have access to those videos.

Search by using the search field at the top of the screen:
or expand the content areas by clicking the plus sign(s) on the left side of the screen (found under Directory):
Add an additional filter by sliding the age filter slider:

More information on WatchKnowLearn can be found here, including the many partners and content contributors:

More information on other video tutorial sites