OpenCulture is a website dedicated to linking to free, open information on the internet. They have links for educational sources including math, foreign language, literature and others. They have sources that are appropriate for a variety of ages, from preschool age - adult.
Kids.gov: The U.S. Government's Official Web Portal for Kids
More than just a website about the U.S. government, this website provides links to a variety of topics. It is most appropriate for elementary-aged kids.
Great Websites for Kids from the American Library Association: They highlight some of the best websites and have the sites organized by topics. They have a wide variety of websites that are useful for kids of all ages.
Library of Congress -
Usborne Quicklinks: Support for the Usborne books
Mind-Fires Academy - Free secular curriculum blog
Discovery Education -
Helping Your Child (US Dept. of Edu.)
197 Educational YouTube Channels You Should Know About | Teachers with Apps
Lesson Plans | POV | PBS
PBS Learning Media
Lessons | TED-Ed
Kids Home Page | National Institute of Environmental Health Sciences