Categorical Imperative

Emmanuel Kant was a German philosopher primarily interested in ethics. Ethics is the philosophy of how to be a good person, and conduct your life in the best possible way. He proposed a series of ideas called the Categorical Imperative, which had three parts.
First, the principle of Universality, which proclaimed that one should only undertake an action if you'd be okay with everyone doing it all the time. So while one could technically get away with taking a $10 tip off a restaurant table while no one was looking, one should not do that unless you'd be okay with people stealing everything not nailed down. (Since most people wouldn't want that, you should not take the money.) I like this idea. (There are other ideas of why not to take the money, from divine command theory, to value theory that proclaims that taking the money would make you a worthless person, to empathy, social contracts, and so on.)
Second, he proposes that other people should be an end, not a means to an end. While these words mean something quite different to philosophers, it's not terribly complex in meaning. Primarily, you cannot ever manipulate people, even for the greater good. No using people, and no lying.
Thirdly, morality is logical in nature, and being evil is irrational. It may be tempting to do evil things, but it will hurt you sooner rather than later, if for no other reason than the reduction of society encourages other people to victimize you. And strength doesn't protect you, because everyone has to sleep sometime. The best implication of this is that morality is empirical, and could be determined by experiment. The most moral behavior would be self-evident when measured.
This comes to some strange combinations, but Kant stuck to his guns in the face of some extremely odd conclusions. He was certain that everyone following his rules would lead to a better universe.
In the face of a number of the crisises occurring now, quite a few of them would not have happened if people had better ethics. It therefore profits us to analyze as many theories as possible.