high school

This engaging strategy game leads players through the parts of the cell and their functions.  Reading is required, and the topics covered are on a middle school and high school level.  There is nothing about the game that is inappropriate for younger kids, except that it's probably too hard.  Here's the description from the developer's website:

There are 2 ways for homeschoolers to use this website: parents can use the included lesson plans (grades 1-12 are represented) to teach about specific topics, or students can explore the virtual "Xpedition Hall" independently and learn about mapping and geography.  The lesson plans are creative and use materials from respected scholars and artists.  The Xpedition Hall requires reading, and is best suited for middle school age or older.  The tasks in the Xpedition Hall emphasize understanding and applying geographic techniques and information, not rote memorization.

Offered free by Microsoft, this web-based software acts as a "virtual telescope" letting you and your homeschol students explore the night sky, and the planets.  The interface was not immediately intuitive to me, and so I'd recommend you begin with a tour (click on the "tour" option on the menu along the top of the screen.)  You can also use the lesson plan provided here: http://www.microsoft.com/education/lessonplans/explorethestars.mspx

The Library of Congress has helpfully linked primary sources, lesson plans, activities and even professional development for teachers on this page.  From here, you can find a wealth of lesson plans, primarily relating to American History, but literature and culture are also well represented.  The lesson plans are built around primary sources-- letters, interviews, and other documents from the time in question.  They are meaty, in depth lessons, from what I've seen.  Most lesson plans are for upper elementary through high school, with a definite bias toward the higher gra