Khan Academy

Students can make use of the extensive video library, interactive challenges, and assessments from any computer with access to the web. Over 3400 videos, covering K-12 math, science topics such as biology, chemistry, and physics, and even reaches into the humanities with playlists on finance and history. Each video is a disgestible chunk, approximately 10 minutes long, and especially purposed for viewing on the computer.

Webmath.com

This site allows you to enter equations and have it solve them and show you the solution! The automated problem solver may include some irrelevant information, but the sophistication of the site, at no charge, is pretty cool. The site covers a wide ranger of math subjects. I used it with some of my son's 7th grade algebra homework, and it was spot on.

Online Math Learning.com

This site provides helpful explanations and illustrations of math concepts in a wide range of math subjects. Some of their help pages include instructional videos.

Alleyoop.com

Alleyoop quickly understands how you learn and guides you to the stuff you need, featuring 1,000s of videos, practice quizzes and challenges from experts like National Science Foundation and NASA eClips™