The (coloured) Mandelbrot set
This applet draws the Mandelbrot set, i.e. the set of points of the complex plane for which the orbit of under the iteration map is bounded, that is, it does not "escape to infinity". It can be shown that the Mandelbrot set corresponds to the set of points for which the corresponding Julia set is connected (i.e. it is not the union of disjoint open subsets).