Khan Academy, started out as one individual making math videos to help explain difficult concepts, it has grown 2,100 videos, and includes exercises and assessment.  The mission, to provide high quality free education to anyone in the world.  Topics focus primarily on math and science from elementary trough college levels. There are other topics related to business, finance and a growing history section. This site is well worth directing your students to or adding to any web enhanced learning environment. Khan Academy is more then just a library of videos visit the Khan Academy about page to learn how students, teachers and parents can track learning.