There is an amazing collection of short videos explaining many, many math and science concepts across the curriculum. These segments are short, focusing on one concept with a few good examples to clarify the lesson.
As one digs deeper into the website, teachers will find a link to "watch" videos, and a link to "practice" concepts taught from those videos. To practice concepts you will need to login with an account, and Khan Acadamy accepts current Google accounts. This becomes a very nice implementation for schools that are integrating Google Apps.
Practice includes answering math questions from a selected level, levels starting at basic arithmetic up through Calculus. Khan Acadamy monitors student progress through their online practice. As a teacher you can sign up as a coach, allowing you to monitor your students progress through their individual practice. You need to have students associate their logins with you as their coach.
Khan Acadamy is great for supplementary materials supporting students that need additional explanations, or those absent from a given class. It is definitely worth the bookmark.