Grigori Perelman is a Russian mathematician, notable for proving the greatest unsolved problem of the 20th century, the Poincaré conjecture. Prior to that work, Perelman became known for giving a short proof of the Soul conjecture.
Perelman is the only person to ever turn down the prestigious Fields medal, and has chosen to leave mathematical work. He posted his solution of the Poincare conjecture on the internet, and it has not been published in peer reviewed journals. Since then, his proof has been reviewed independently by other mathematicians who have published versions of his proof in peer-reviewed journals. In June 2010 he rejected the millennium prize, valued in one million dollars, and gave no explanation.
The New York Times says Perelman "has declined previous mathematical prizes and has turned down job offers from Princeton, Stanford and other universities."