The interactive illustrations are brilliant! I've seen a few writers use these and for me it helps to grok the concepts quickly. If only school curriculum could incorporate more tools for learning like this, I think more kids would find interest in science and engineering.
ciechanow.ski is one of the great treasures of the Internet: this isn't the only one, there's like a dozen more as good or better.
I think about the mechanical watch one at least once a week!