John Rawls is probably the most important political philosopher of the 20th century, especially for his best known work, A Theory of Justice. In it, he describes his theory of the "original position", where justice is conceived as how would we agree to form a society based on a position of neutrality. In asks us to go behind a "veil of ignorance", that is to say, imagine that we didn't know what position we would take in society. We wouldn't know if we were rich or poor, nor would we know our capabilities, such as our intelligence or work ethic. We wouldn't even know what beliefs we would have (i.e. if we were Christians or Atheists, etc). We would still know scientific facts and facts about humanity in general, however. From this "original position" we would have to come up with a way to form society, and it would be a more just society, since we couldn't design it to favor any one group. So for example, no one would choose to make slavery legal, because they wouldn't want to risk being a slave.
[Two of the conclusions] that he drew from this were that everyone would be guaranteed basic liberties. In other words, unlike basic utilitarianism, even if taking away a liberty resulted in greater total happiness, no one would want to risk being the one without the liberty (such as slavery). In addition, while social and economic difference may occur as a matter of practicality (such as that better waiters are given more to encourage effort on the part of the waiters), they must be to the advantage of the least well off. You can read more in the Stanford Encyclopedia link about why he believes this is the case.
Robert Nozick was a contemporary of Rawls (in fact they both taught at Harvard at the same time). He took Rawls's A Theory of Justice further in the anti-consequentialism direction, saying that the outcome of a given society isn't enough to determine if it is just, you must look at what led to that outcome. Even if someone gets fantastically rich, if they do so from the free decisions of adults, that disparity is still just. He favor a minimal state, and claimed that taxation for the purpose of wealth redistribution was part time slavery. He was also super handsome. Seriously, look him up on Google images.
Karl Marx was a 19th century Marxist philosopher, best known for his Marxist political viewpoints.