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HERE ARE SOME CONTRIBUTIONS:

EVOLUTION
Charles Darwin published his treatise On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life on November 24, 1859. The book sold out the first day and immediately was both attacked and praised.
The idea itself of evolution was not novel, but nobody had ever proposed a mechanism that could explain it. Darwin's contribution to our understanding of evolution was the Principle of Natural Selection. (Alfred Wallace proposed the same ideas simultaneously, but became less famous).
The idea that species form when two parts of a population are genetically isolated; that the environment shapes the species; that there is no need to postulate a designer in order to explain the structure of an organism; that complex knowledge can be acquired by mutations and selection; and that it can be passed over through the generation (note: DNA was yet to be discovered) had a long lasting impact on the history of thought, of science, of religion and of philosophy.

SUBCONSCIOUS
The main contribution of Sigmund Freud is the idea that some very important mental activity is not only subconscious, but firmly resists conscious access through the mechanism of repression. At first Freud's idea of the unconscious was greeted with consternation as being virtually self-contradictory, but it has since won acceptance as being useful and entirely feasible.
The impact of this notion on our self image has been enormous ...

NUCLEAR FISSION
Since the beginning of the XX century, physics had become the queen of sciences. It had become so advanced and specialized, that a scientist had to choose between a theoretical and an experimental career.
Enrico Fermi, an Italian American born in Rome in 1901, was the last great physicist to do both. His theory of beta decay introduced the last of the four basic forces known in nature (gravity, electromagnetism and two nuclear forces). But most importantly, he also invented and designed the first nuclear reactor, starting it up in a historic secret experiment at the University of Chicago on Dec. 2, 1942.
If Hitler had not hounded Jewish scientists out of Europe, the Anglo-American atom bomb program sparked by the discovery of fission late in 1938 would have found itself shorthanded. Fermi and his fellow emigres--Hungarians Leo Szilard, Eugene Wigner, John von Neumann and Edward Teller, German Hans Bethe--formed the heart of the bomb squad. In 1939, still officially enemy aliens, Fermi and Szilard co-invented the nuclear reactor at Columbia University.
This technology changed the history of the XX century, and probably also the early XXI.

INFORMATION AND COMPUTATION
The XIX century belonged to chemistry, the XX to physics, and the XXI to information (in every form).
Claude Shannon and Alan Turing separately created the conceptual framework that enables us to talk about information and computation, the central concepts of XXI century (together with genomics, of course).
Turing simply created theoretical computer science by defining the concept of Turing Machine, and formalizing the notion of computation. Then moved on to creating computer engineering, building what probably was the first electronic computer of history, during the second world war.
Shannon formalized the notion of information, creating a mathematical theory for communication and storage of information, based on probability and the notion of entropy.

EVOLUTION OF LANGUAGES
The first systematic theory of the relationships between human languages began when Sir William Jones, "Oriental Jones," proposed in 1788 that Greek and Latin, the classical languages of Europe, and Sanskrit, the classical language of India, had all descended from a common source. This became the theory of "Indo-European" languages, and today the hypothetical language that would be the common source for all Indo-European languages is called "Proto-Indo-European". It is now widely accepted that all languages are formed by evolution from proto-languages, and that they are organized in families that reflect their origins. More controversial is the hypothesis that possibly all languages might ultimately originate from a single mother tongue.

FALSIFIABILTY
The demarcation problem, or definining what distinguishes science from non-science, was solved by Karl Popper in the XX century. The notion that scientific knowledge cannot be 'verified' but only 'falsified' paves the way both to a demarcation between science a non-science (knowledge that cannot be falsified is non scientific), and to a depper understanding of the epistemological status of knowledge and scientific method: the first one temporary and fallible; the second one devoted to the effort to falsify existing theories. One of the most influential philosophers, loved by scientists, achieved the result of imposing his ideas as 'obvious' to such an extent that their greatness is today non fully acknowledged. Turned our way to perceive knowledge and science completely on its head.