Cambridge Histories Online: Philosophy:

Cambridge History of Hellenistic Philosophy, by Keimpe Algra, Jonathan Barnes, Jaap Mansfeld and Malcolm Schofield, 1999

Cambridge History of Later Greek and Early Medieval Philosophy, by A. H. Armstrong, 1967

Cambridge History of Later Medieval Philosophy: From the Rediscovery of Aristotle to the Disintegration of Scholasticism 1100–1600, by Norman Kretzmann, Anthony Kenny, Jan Pinborg and Eleonore Stump, 1982

Cambridge History of Renaissance Philosophy, by Charles B. Schmitt, Quentin Skinner, Eckhard Kessler and Jill Kraye, 1988

Cambridge History of Seventeenth-Century Philosophy, by Daniel Garber, Michael Ayers, Roger Ariew and Alan Gabbey, 1998

Cambridge History of Eighteenth-Century Philosophy, by Knud Haakonssen, 2006

Cambridge History of Philosophy 1870–1945, by Thomas Baldwin, 2003

Cambridge History of Jewish Philosophy Vol. 1. From Antiquity through the Seventeenth Century, by Steven Nadler and T. M. Rudavsky, 2009

The Cambridge Dictionary of Philosophy, 1st edition. Robert Audi, general editor. Cambridge University Press, 1995.

Ron Blazek and Elizabeth Smith Aversa's The Humanities: a Selective Guide to Information Resources (2000; 50) lists this work as among the best one-volume philosophical dictionaries. There are thousands of entries listed alphabetically on a wide range of philosophers and topics. Widener: RR 4093.29

Encyclopedia of medieval philosophy: philosophy between 500 and 1500, ed. by Henrik Lagerlund. Dordrecht; NY: Springer, 2011.

The Encyclopedia of Philosophy. Don Borchert, ed. 2nd edition. 10 vols., 2006.

The Encyclopedia of Philosophy, although largely superseded by the Routledge Encyclopedia of Philosophy, is still valuable for looking up background and introductory material on a philosopher or philosophical topic. Widener: RR4903.4; Robbins: B41.E5

International Encyclopedia of the Social and Behavioral Sciences (IESBS)

The IESBS aims to be the most comprehensive and up-to-date resource covering a variety of topics in the social and behavioral sciences. It covers a wide variety of topics in the areas of mind, cognition, perception, and the like that are relevant to philosophy. The material contained in the IESBS tends to reflect current trends and research in the social sciences. However, this sometimes means that articles carry a certain bias or slant towards the more favored current theories, and may not treat a topic as fully or as objectively as might be expected. Nevertheless, IESBS articles are signed, dated, updated often, and contain references and links to further resources. The user can retrieve articles by summary, HTML full-text, and in PDF format.

Islamic Philosophy Online

This site is devoted to Islamic philosophy, and presents many full-text books, articles, and other resources for those interested in researching this field. 

Meta-Encyclopedia of Philosophy

This site compares the treatment of subjects and topics in six philosophy encyclopedias on the Internet. In other words, it lists subjects and topics alphabetically, and marks on a grid if and where various Internet philosophy encyclopedias cover these. There is no indication as to when or how often the site owner updates the information. 

Perseus Digital Library

The Perseus Digital Library is one of the premier sites to find texts from and resources for doing research in the Classical era. In addition to primary texts, users will find dictionaries, commentaries, maps, and art.

Philosophical Lexicon

The Philosophical Lexicon is a humorous dictionary of “philosophical” terms. (Please note that this dictionary is for entertainment purposes only!)

Pragmatism Cybrary

This site provides information about and links to resources on the figures and thought of Pragmatism.

Routledge Encyclopedia of Philosophy. General Editor, Edward Craig. Routledge Keegan Paul, 1998. Ten volumes.

This encyclopedia is the print successor to the Encyclopedia of Philosophy, listed above. The work gives alphabetical entries about philosophers and topics, with commentary and bibliographies. Widener: RR 4903.45; Robbins: Reference Section, B51.R68 1998

Stanford Encyclopedia of Philosophy

The Stanford Encyclopedia of Philosophy is an electronic encyclopedia of philosophy, with entries on philosophers and philosophical topics written by prominent scholars in the field. Entries are signed and dated, and include bibliographies, Internet resources (where applicable), and cross-references to related entries.