Benoit Mandelbrot, who died last week at 85, was to math what Carl Sagan was to astrophysics. He wasn't just a researcher; he popularized scientific thought. And he's best known for bringing fractal mathematics to the masses.
Mandelbrot's experiments with number sets and computers in the 1970s led to his discovery that you could draw geometric shapes that were "self similar," which is to say each of their parts shares similarities with the whole. (This is why, for example, when you look at a Mandelbrot fractal, you see the same shapes emerging at its edges as you zoom into it.) His great insight was to group a number of similar kinds of mathematical phenomena together and identify them all as part of fractal mathematics. By using fractals, mathematicians and physicists could much more easily explain "rough" shapes in the real world, ranging from mountain ranges to the shapes of trees. They could also simulate those shapes too.
In 1982, Mandelbrot published The Fractal Geometry of Nature, which popularized the idea the natural world was organized by elegant, mathematical principles that could be predicted. He worked tirelessly to make his work accessible to a broad audience, which is why the fractal is perhaps one of the most widely-recognized mathematical ideas of the past half-century. Nearly everyone can recognize a fractal when they see it. Here are some gorgeous examples of fractal art, from the Mandelbrot Fractal Art Contest.