This is the premier (and free) online-only philosophy encyclopedia with an impressive group of over 100 subject editors from around the world. The executive editors are Edward Zalta and Uri Nodelman from Stanford, and Colin Allen at Indiana University.
"The purpose of the IEP is to provide detailed, scholarly information on key topics and philosophers in all areas of philosophy. The Encyclopedia’s articles are written with the intention that most of the article can be understood by advanced undergraduates majoring in philosophy and by other scholars who are not working in the field covered by that article. The IEP articles are written by experts but not for experts in analogy to the way the Scientific American magazine is written by scientific experts but not primarily for scientific experts." Source
Routledge Encyclopedia of Philosophy by Craig, Edward, ed.
Call Number: RR 100 R765c
Together with Borchert's Encyclopedia of Philosophy (below), the most respected general reference work in the discipline. Here's where to identify the core works and authors on a topic.
Encyclopedia of Philosophy by Borchert, Donald M., ed.
Call Number: 103 En193, Ed.2
Comparable to the Routledge set in terms of coverage and depth, this is the 2nd edition of this classic reference work from 1967. Many of the original articles appear in the 2nd edition, attesting to their quality and importance.
Encyclopedia of the Enlightenment by Kors, Alan Charles, ed.
Call Number: RR 940.25 En19k
If your philosophic topic or person is of this period, an entry in this encyclopedia may provide more detail and context than an encyclopedia that covers all periods.