MIT (Massachusetts Institute of Technology) has been highlighted
here on FreelyEducate.com before, for their 1,900 (!) courses available
for free (!) at the college level.
Well, now they're focusing on high schoolers. Check out the MIT OpenCourseware: Highlights for High School website for the course listings.
Here is a direct link to the Teachers page, a great place to start. You can watch a 3-minute video tour of the site here, which outlines all the site has to offer teachers of high school students.