Loopy lets you create interactive simulations like the one below – just by drawing.

This is how they describe themselves:

It’s the ancient, time-honored way of learning: messing around and seeing what happens. Play with simulations to ask “what if” questions, and get an intuition for how the system works!

Raw code is too inaccessible. Also drag-and-drop is too mainstream. But with LOOPY, you can model systems by simply drawing circles & arrows, like a wee baby

I don’t quite get it, but I suspect some readers might find it useful. Thanks to Flowing Data for the tip.