Cambridge Dictionary

Columbia Encyclopedia

Dictionary of Philosophical Terms and Names

Internet Encycolopedia of Philosophy - James Fieser, Ph.D., and Bradley Dowden, Ph.D.

MacTutor History of Mathematics archive

Philosophy Documentation Center - Established in 1966, the Center has maintained an exclusive focus on the development of innovative reference products, instructional materials, and publishing services for the philosophical community.

Stanford Encyclopedia of Philosophy - see also: InPho project, a dynamic formal ontology for the discipline of philosophy. The site allows you to browse the taxonomy of Stanford Encyclopedia of Philosophy

Wikipedia - Philosophy