Philosophy
For me, philosophy is the only thing in life that really has any meaning. While on some level it is all hypothetical and academic and of no use in the real world, on another level it is the original and ultimate source of our understanding of ourselves and the world we are in.
The influence of philosophy may not be obvious, and it is rarely immediate, but it is all-pervading. In the western world in the 21st century, we view democracy as a good thing, the rights of the individual as paramount, and all people as equal. A hundred years ago, the world was a far more heirarchically authoritarian place. What changed was that our ideas about rights, ethics and equality changed. The study of rights, ethics and equality are all firmly within the field of philosophy.
In these modern times, the debates are about animal rights, genetics and cloning, and abortion. A hundred years from now, our world will have been shaped by what philosophers conclude today.