The Stanford Encyclopedia of Philosophy is a freely available web resource that first came into being nearly 20 years ago (which makes it positively ancient in online terms).
It includes full definitions of a number of terms of relevance to Law students, such as Legal Positivism, Natural Law Theory & Constitutionalism. There are also entries on individuals such as Austin, Rawls, Levinas and Habermas.
Recommended for those wanting to gain an overview of an individual or particular philosophical approach.
Information on the Editorial Board and how the entries are added / updated available here.