Khan Academy has done a very good job of adding free response questions to AP Chemistry. I want the exact same thing done for AP World History and possibly other AP Subjects. 

I was also looking for more engineering content on Khan Academy, especially topics like Robotics, Programming etc...

Maybe you can team up with the FIrst Robotics Competition. (Its a competition performed by thousands of kids throughout the country).

Other than that Khan Academy is really helpful and is doing a really good job.