Although I'm no longer a part of it, I still love this web site for students in grades K-12: www.gmability.com/education.
When you go there, you get a very standard GM page, which is a bit cumbersome. But, I encourage you to follow the links to one of the grade ranges (K-4, 5-8 or 9-12). You'll find some great games and stories written for each grade level. The stories focus on science, energy and the environment, and scratch the surface of some fun technologies, like 3D modelling and satellites. The games are all directly related to the content, and reinforce ideas introduced in the articles.
The high points of this site are the interactive tours. When you read an article on fuel cells, you can click the "interactive tour" button and get an in-depth illustration of how the hydrogen molecules react with O2 to create electricity. You can do the same for an inside look at the internal combustion engine.
I must admit, I was shocked when I first learned about this site. Of course, the way I learned of it was interviewing for the job of creating and managing its content! We've struggled for almost three years to publicize this site without an all-out advertising blitz. The strange thing is that every time we put it in front of teachers, they rave about it and say they would use it in their classrooms. What's the barrier to making this happen?