Lissajous curves and related curves, along with their automated creating apparatus, such as Spirographs and Guilloché patterns.
I participated remotely in Instanssi 2016 with a quick Processing entry. It placed #3/3. I had scribbled together something resembling a classic Amiga effect, known as shadebobs, but it ran sub-1 frame per second on my netbook. So, to get a video capture out of it, and to support the event, I entered it in the compo at Instanssi. It was accepted into the "Pikkiriikkinen demo"/Intro competition, with the size only including the source file, which could've been squeezed down even more by making all the variable names single characters, but I couldn't be bothered. The standalone Linux application exported from Processing is ~4.3 MB, and even this needs the Java Runtime Environment, so the admittance into the Intro category is sketchy (geddit?), but then, the border between an intro and the environment always has been.
Anyway, turns out the effect is slow even on proper machines, so enjoy this still image of an effect that was meant to be animated.