10 Excellent web sites for teaching?

This seems like a seemingly simple question for teachers:

Could you identify 10 excellent web sites for your grade level / subject area?

Ideally, of course, teachers would know 10 or so excellent sites for each unit, not just for the overall course that they’re teaching. After all, the Internet has been around for most people for at least a decade now and there are an incredible number of valuable resources on almost any topic.

And yet I’m guessing that many (most?) teachers would have trouble answering even the simple question above.

Scott poses a great question. I’ll have to work on mine. I know the first one on the list is PhET: Interactive Simulations by the University of Colorado at Boulder. I’ll post my top ten for physics when I finish them.