The rest of the Rights Philosophy Forum is dedicated specifically to the philosophy of ethics and human rights. But that's hardly an isolated area. Most philosophers rightly insist that the areas of philosophy are incredibly interconnected. Ethical principles may be based in turn on principles of metaphysics or the philosophy of mind. Often, what one is willing to "accept" as a premise in ethics is precisely what someone else is debating hotly in epistemology.
First, it might help to have a look at some of the major branches of philosophy (there are several other specialties, and sub-fields of specialties, as well):
Now you might want to check out some of the classic philosophers, in order of appearance:
Note: some of these philosophers are also listed on the Top 20 page of the RPF, the difference being that the Top 20 section focuses primarily on their ethical and political views which have influenced rights philosophy. Also note: there are obviously several philosophers not mentioned; I've tried to only list the most influential/most-often mentioned to make this list more accessible.