"In 1905 Albert Einstein published a paper that explained experimental data from the photoelectric effect as being the result of light energy being carried in discrete quantized packets. This discovery led to the quantum revolution."

So, he basically described photons as a particle for the first time. That's also ridiculously huge. http://en.wikipedia.org/wiki/Photoelectric_effect Considering they had been generally thought of as a wave before that. (http://en.wikipedia.org/wiki/Maxwell%27s_equations) Setting the stage for a lot of wave / particle duality aka QM theory.