Enrico Fermi

Worked together with Max Born. He also discovered Fermi Statistics, the statistical laws for governing the elementary particles - Fermions - affected by the Pauli Principle. He was also the brain behind the first controlled nuclear chain reaction.

"for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons"

-- Nobel Prize motivation

source: http://www.nobel.se

On one of the nuclear bomb tests, everyone had a ton of monitoring equipment set up to measure the output of the detonation. Fermi thought about it for a while, and just before the explosion, he tore up a sheet of paper into confetti. When the shockwave hit the bunker, Enrico dropped the paper. He measured how far back they were from where he dropped them. He did some calculations and in fifteen minutes calculated the approximate yield of the bomb. He was laughed at when he announced it, but when the results came in from the measurement equipment, he was correct.

In a physics class he was teaching, Fermi asked the students to pick a profession at random. Someone came up with piano tuners. He then asked someone to pick a city, and someone said Pittsburgh. Using logic and assumptions, he figured out how many piano tuners there were in Pittsburgh. He was slightly off... he guessed 106 and there were 115.