MIT (Massachusetts Institute of Technology) has been highlighted here on Freely Educate recently, for their 2,000 free college courses.
Did you know they also offer courses for high schoolers? The full courses offered are in the Math, Science, and Humanities subjects.
Here is a direct link to the Teachers page, a great place to start. You can watch a 3-minute video tour, which outlines all the site has to offer teachers of high school students.
How to Get this Freebie: Check out the MIT OpenCourseware: Highlights for High School website for the course listings.