Building upon the theme of playable simulations, I recommend Nicky Case's entire website. The complex systems simulation is my favorite, but I also recommend this interactive comic on the science of memory, and this one about game theory, which is inspired by a book called The Evolution of Trust, which I summarized here. If nothing else, these visualizations are a window into the future of online writing.

5/ A great little game that uses game theory to help explain our epidemic of distrust and how we can fix it.