Benoit Mandelbrot (1924-2010).
I never met him, but his ideas enriched my life. I remember spending countless hours playing with Fractint (for DOS) - it was a great stress relief to go wandering around inside the secret corners of equations (or designing new ones), and many such images graced my desktop for a time (sometimes manipulated further by PovRay, sometimes as-is). Later I encountered more mature presentations and extensions of his ideas in Gleick's 「Chaos」, connecting it to both Lovelock's Gaia Theory and Jupiter's Red Spot (metastability!). With a powerful idea, some parts of the world suddenly make more sense, and Mandelbrot's gift to humanity is that his ideas were large - I know I am not alone in appreciating the trailhead he gave us.
I'm not sure how long his webpage will remain up, so visit while you can.