The following sites contain videos that have educational content.  These are video sharing sites, so keep in mind that you may encounter some that are not very useful, and perhaps some that are not school appropriate.  In fact, many of these sites may be blocked by Internet filters in a school district, but these are sites that are geared specifically for offering educational videos. These can be useful for parents helping with homework at home, or for teachers to find useful videos that they can download to their computer, bring them to school, and play for the students in class.

Meta Search Tools for locating videos:
These two sites will search over 100 different video sharing sites and give you access to the results in one convenient window.

http://www.videofetcher.com
http://www.metatube.net

Video Sharing Sites with Educational Content:
These sites are designed specifically to share videos that are educational and appropriate for K-12 students.

Khan Academy
http://www.khanacademy.org
This non-profit organization provides about 2000 instructional videos in Math, Science, History, Finance, and Test Prep.

WatchKnow
http://www.watchknow.org

TeacherTube
http://www.teachertube.com

Teachers Domain (PBS Videos!)
http://www.teachersdomain.org

School Tube
http://www.schooltube.com/

Teacher Zone
http://teacher.tenmarks.com/
The most comprehensive library of math video lessons on the planet!

Math Tube
http://www.mathatube.co

MathTV
http://mathtv.com/

Science Tube
http://www.science-tube.com

Untamed Science
http://www.untamedscience.com

NeoK12
http://www.neok12.com

Kids Know It Network
http://www.kidsknowit.com/interactive-educational-movies/index.php

The Futures Channel
http://www.thefutureschannel.com

Teachers.TV
http://www.teachers.tv

iTunes U (this site has an introduction, but iTunes U requires iTunes on Mac or PC)
http://www.apple.com/education/itunesu_mobilelearning/itunesu.html

School Tube
http://www.schooltube.com

Schools Tube (yes, this is different than the one above)
http://www.schoolstube.com

Next Vista for Learning
http://www.nextvista.org

How Stuff Works
http://www.howstuffworks.com

School Wax TV
http://www.schoolwaxtv.com

EduBlogs TV
http://www.edublogs.tv

Nibipedia (this is what you would get if you combined Wikipedia and YouTube - a video for every entry!)
http://nibipedia.com

DotSub (good resource for foreign language teachers, ESL/EFL teachers, and teachers of students with hearing impairments)
http://dotsub.com/

The following three sites have videos of experts discussing important current issues:

Big Think
http://www.bigthink.com

TED - Ideas Worth Spreading
http://www.ted.com/index.php

ForaTV
http://fora.tv

The following two sites have large collections of professionally-produced programs:

Hulu (a great source for TV news shows, TV shows and movie clips)
http://www.hulu.com

Snag Films (collections of documentaries from producers like National Geographic)
http://snagfilms.com

The following sites have much of the same content that can be seen on their television stations:

The History Channel
http://www.history.com

The Discover Channel
http://dsc.discovery.com

CNN Student News
http://www.cnn.com/studentnews

iCue (presented by NBC News)
http://www.icue.com