Alexander Grothendieck and the Crafoord prize
By all accounts, Alexander Grothendieck is one of the most brilliant mathematicians of all times. His ideas shaped mathematics of the 20th century and led to the solution of a number of long standing conjectures. He worked in diverse fields but he is mostly known for his work in Algebraic Geometry and Homological Algebra.
Born in 1928, he worked for many years in Paris where he established the modern school of Algebraic Geometry. Nobody would question his mathematical talent, but his idiosyncratic personality often made him the object of criticism by his peers. For others it is exactly his personality that made him more fascinating.

Liberal by nature, he felt that funding from military organizations had no position in Mathematical Research. He declined invitations to conferences sponsored by what he considered unsuitable sources. And he took a stand both against the Western and the Eastern block. He is known to have lectured on Homological Algebra in the forests surrounding Hanoi in order to protest the Vietnam war.

Finally as he was retreating from active research, he accepted a position at a small college in France. It was at that time in 1988 when the Swedish Academy of Sciences decided to award him and Pierre Deligne the Crafoord Prize for their work in Algebraic Geometry. This prize had a monetary value of over $400,000.
Here are some interesting excerpts from his letter of decline:

"...My salary as professor, even my pension starting next October, is more than sufficient for my own material needs as well as those of my dependents; hence I have no need for money. As for the distinction given to some of my work on foundations, I am convinced that time is the only decisive test for the fertility of new ideas, or views. Fertility is measured by offspring, not by honors."

"But is it not clear that superabundance for some is only possible at the cost of the needs of others?"

"...agreeing to participate in the game of prizes and rewards would also mean giving my approval to a spirit and trend in the scientific world that I vew as being fundamentally unhealthy, and moreover condemned to disappear soon, so suicidal are this spirit and trend, spiritually and even intellectually and materially."