I don't know if this justifies the SHOW HN tag: I guess I just finished a weirdly over-engineered interface to animate prime numbers drawn to a spiral with lots of configurable parameters.
It does not really work on mobile browsers. On a desktop you'd see a control area on the left, that can be hidden by pressing enter or you change the alpha value. And behind this there's the canvas with representing the spiral and some arcs, as you define them on the right. Pressing "ESC" starts the animation (there's also a checkbox on the top). The parameters will change the spiral in real-time.
Have fun =)