The
Mandelbrot set is his discovery, and serves as a sort of
index into the
Julia set. The set (whose
border can only be
approximated), the values of C for which Z'=Z^2+C never diverge, happens to also be the set of
hyperplanes on which there are non-diverging values in the respective Julia.
Mandelbrot has also done much more than that one fractal; he has written many a treatise on the fractal nature of nature, for example.
blaaf: Thanks for pointing out my inappropriate word choice. I've fixed it. You can stop flaming me now.