In his library John D. Rockefeller, Jr. kept a well-thumbed copy, with notes written in it, of Marx's Das Kapital* (Zeese Papanikolas, Buried Unsung, University of Utah Press, 1982, p. 133). You can bet that Rockefeller did everything possible to prevent his workers getting access to this work, though. E.g. at Rockefeller's Colorado Fuel and Iron company, in Ludlow, Colorado (later site of the infamous Ludlow Massacre by company-hired thugs of striking workers' families), the company town's company-owned library was censored by company officials; no left-leaning literature was permitted (George S. McGovern and Leonard F. Guttridge, The Great Coalfield War, University Press of Colorado, 1984, p. 26).
"Das Kapital": German for "capital," as in the economic term for the money that one invests, or makes interest on. (Not "capitol" as in the official city of a country, not "capital" as in upper-case letters, not "capital" as in capital punishment or capital crime. The etymology all these share is the Latin word "caput" or its adjective form, "capitalis," which means "head." The Romans used it metaphorically as we do, for "leader," "center," "most important." Economically, one's "capital" is more important than the interest or dividends one makes from it.) Thus supporters of the private investment stock-market system are called "capitalists." Note that capitalism is not identical to "the free market," though people often erroneously conflate them. Marx was opposed to capitalism in any form, including its free market version, which he regarded as not really free. Some recent socialists, though, have experimented with incorporating some free market characteristics into something they call "market socialism."
Yep, many people have found a Marxist analysis of economics profoundly helpful, whether they were using the analysis towards Marx's general aims of social justice, or, in the case of Rockefeller, against Marx's aims and for personal profit.
Makes one wonder why most of the U.S. educational system and media regard Marx's writings as too "dangerous" (or "wrongheaded," or "out-dated," etc.) to be made available to the general public.
Marx's main insight could be paraphrased as the cynical motto which has, since Marx's death, become commonplace:
To understand any human situation, follow the money.
And since, according to Marx, economic power (i.e. wealth) is the driving force for the rest of the situation,
to tweak any situation, or any ideology, tweak the money. (Thus Marx thinks that social injustices such as sexism and racism are caused by an underlying unjust economic situation. As we'll see later, Gerda Lerner disagrees, and thinks that it's the other way around, that exploitative economics is based on patriarchy, i.e. sexism.)
Add to this Marx's contention that capitalism is inherently unfair, that the people making the wealth are those not benefiting from it, and his desire for people to adjust the money system for the benefit of humanity, rather than having people adjusted for the benefit of the money system, and you've got a lot of Marx in a nutshell.
Keep in mind that you needn't accept Marx's own particular prescription for what to do about the problems he diagnoses, even if you do make use of his analysis, his description, his diagnosis, of what's happening. (Though I hope you don't make use of the knowledge as Rockefeller did, to exploit people even more.)
For example, Marx predicted that powerful capitalists (i.e. mega-stockholders) would never voluntarily give up any of their power. (E.g. charity would occur, but then it would occur in such a way as to cement the position of the very powerful, not in a way that could lead to real power-sharing. E.g. the wealthy could set up high-profile charities for children – and feel good and have good PR -- while lowering the wages of their fathers to compensate, chipping away at the legal rights of the poor, and even simultaneously making them feel dependent on the wealthy, and possibly even grateful for the charity.)
Thus, Marx notoriously concluded that economic justice (and the social justice that he thought was predicated on it) would never be achieved by peaceable means, but only by violent revolution. (Many people writing in the decades after the French revolution of 1789 thought this, sadly and bitterly, but immovably.)
Marx's prediction that violent revolution was necessary was disproven – to the astonishment and elation of Marxists and many progressives worldwide – when Salvador Allende won Chile's presidential election in 1970. On the other hand, both before and after the election, the U.S.'s Nixon administration and other outside powers poured money and other resources in to defeat Allende and his allies. (This was during the Cold War, and Allende was accepting resources from Cuba's Fidel Castro, as well; Allende also nationalized Chile's copper mines back from their American owners.) The conflicts in Chile increased until a CIA-backed military coup in 1973 ended Allende's presidency. Allende himself apparently committed suicide. Allende's Marxist goals were not able to be implemented, but his electoral victory – as a Marxist -- had nonetheless disproven one of Marx's own dogmas, and one of the most repugnant. Marx's "prescription" of what to do (revolution) has since been cheerfully abandoned by many people who are in other respects heavily influenced by him.