Secular Charlotte Mason

SecularCM is not responsible for any of the content on these sites and recommends that parents preview the sites before using them with children.

Videos and lectures from various colleges and universities.

Free science library and genetics site.

YouTube type videos/lessons for and by teachers. 

Preview before turning the kids on this site.

Many of the videos are available elsewhere on the internet but the key feature of this site is how easy it is to search for particular topics and/or ages. 

Short online educational videos and lessons for just about every age and every subject.

This site seems to link to just about every online course and learning resource available including PBS shows and lessons, LibraVox, podcasts and online encyclopedias.  Scroll down the page to find open courseware from MIT, Notre Dame, Tufts, Utah State, etc.

Online Education Database

 I've not had time to check this site out as thoroughly as I like but the lectures look like they would make nice supplements for your high school studies. Looks like they also offer courses but I haven't looked at any of them.

Academic Earth

Interesting talks and lectures by well-known people from all walks of life. For a general audience so you may want to check it out before turning the kids loose on the site. 

TedTalks