"[S]ince 1967, the Encyclopedia of Philosophy, described by Booklist as 'the best and most comprehensive English-language reference source for philosophy,' has been the cornerstone of the philosophy reference shelf." A great place to begin your research or gather authoritative background information on a topic.
"From its inception, the Stanford Encyclopedia of Philosophy was designed so that each entry is maintained and kept up to date by an expert or group of experts in the field. All entries and substantive updates are refereed by the members of a distinguished Editorial Board before they are made public. Consequently, our dynamic reference work maintains academic standards while evolving and adapting in response to new research." Freely available on the open web.