Gene Callahan has a list of the great ideas of social science here, and he solicits other ideas.
I might add:
Division of labor and its social ramifications (Smith, Durkheim, Marx)
Externalities and the impact of property rights on social organization (Pigou, ? somebody had to have said this before him)
Law of one price (Cassel, Keynes, again there have to be other earlier examples)
Double consciousness (DuBois)
Non-neutrality of money (Keynes, Hayek, there have to be earlier examples)
Social construction of reality (Berger, Luckmann)
I was also intrigued by this comment by Greg Ransom:
"The most important idea – on a par with Darwin’s directly parallel
idea – is the identification of an explanatory PROBLEM in the
design-like order of the rough, undersigned & non-dictated
coordination of individual plans in the great market society, and the
solution to this problem in the bottom-up mechanism of learning in the
context of changing relative prices, negative rules of just conduct, and
individual judgments of local conditions.
Until this ideas was together in one piece by F. A. Hayek economics,
like biology before Darwin, was just a miscellany of components —
afterwards everything makes sense"
That's funny. I would have thought that's the primary reason why Adam Smith is considered a sort of father of economic science, even though he certainly wasn't the first to write on the subject. And much as I like Hayek, Smith beat him to it by over one hundred and fifty years. Indeed, Darwin found inspiration in a lot of the classical economists. Just an important thing to keep in mind.