We're just gonna bet one big misconception out of the way right off the bat: sure, Simone de Beauvoir was the very long-time BFF of that hotshot existentialist Jean-Paul Sartre, but that doesn't mean she played second fiddle among the intelligentsia. (In fact, the word is that she bested him on her postgraduate examination at the École Normale Supérieure—basically the Super Duper Harvard of France—but the jury granted Sartre the coveted first place spot instead of Beauvoir, who then placed second.) This girl could hold her own, even when the world was favoring her male partner in crime.
For a long time Beauvoir struggled for recognition largely because she spent so much time promoting Sartre's ideas. But here's the thing: she had some awesome ideas all her own. (The problem was, she didn't really see herself as a philosopher.) Quickly realizing that women didn't have it so great, she wrote an encyclopedic work called The Second Sex. This bible of feminist thought argues, as the title suggests, that women are treated as second-class citizens.
But Beauvoir did much more than spark the flames of 20th-century feminist theory, which is no small task to begin with. She also wrote about ethics, politics, existentialism, and phenomenology. Plus she was a hardcore activist, taking to the streets on issues such as abortion, the French colonial occupation of Algiers, and the student rallies of 1968. Add to that the fact that she also wrote some great novels, and you've got a recipe for one kick-butt lady.
Beauvoir started out as a high school teacher, but pretty quickly became a public intellectual on a huge scale. By in the time she died in 1980, she had made irrevocable transformations to women's status in society. Her assertion "that women is not born, but made" was and remains the definitive declaration of woman's lib.