Paul Stamets for the Chemistry or Medicine/Physiology.

Check out his work with fungi. He found mushrooms to kill MSRA superbugs 100x better than known drugs, clean up toxic waste sites restoring life and habitat, filtering streams from agricultural and factory farm runoff, produce a viable source of biofuel, alternative to chemical pest controls, etc. Ver good video and awesome research done by this guy.

TED.com video:
6 Ways Mushrooms Can Save The World