The Jewish Encyclopedia was an encyclopedia originally published between 1901 and 1906 by Funk and Wagnalls. It contained over 15,000 articles in 12 volumes on the history and then-current state of Judaism and the Jews as of 1901. It is now a public domain resource.
Jenny Mendelsohn, of University of Toronto Libraries, in an online guide to major sources of information about Jews and Judaism says of this work, "Although published in the early 1900s, this was a work highly regarded for its scholarship. Much of the material is still of value to researchers in Jewish History."  Reform Jewish rabbi Joshua L. Segal calls it, "a remarkable piece of Jewish scholarship" and adds, "For events prior to 1900, it is considered to offer a level of scholarship superior to either of the more recent Jewish Encyclopedias written in English." 
The unedited text of the original can be found on the Web at the website listed in the "External links" section of this article. The site offers both excellent JPEG facsimiles of the original articles and very precise Unicode transcriptions of all texts.
The search capability is somewhat handicapped by the decision to maintain all diacritical marks in the transliterated Hebrew and Aramaic from the 1901–1906 text, which used a large number of diacriticals not in common use today. Thus, for example, to successfully search for "Halizah" (the ceremony by which the widow of a brother who has died childless released her brother-in-law from the obligation of marrying her), one would have to know that they have transliterated this as "Ḥaliẓah". The alphabetic index ignores diacriticals so it can be more useful when searching for an article whose title is known.
The scholarly apparatus of citation is thorough, but can be a bit daunting to contemporary users. For example, the names of the tractates of the Talmud are generally abbreviated; it is left to the reader to know that "Sanh." is Sanhedrin a tractate in the order of Nezikin or that "Git." is Gittin, a tractate in Nashim. Citations of modern works can be equally cryptic: the reader is expected to know that "R. E. J." is Revue des études juives; books that might have been widely known among scholars of Judaism at the time the encyclopedia was written (but which are quite obscure to a lay reader today) are referred to by author and title, but with no publication information and often without indication of the language in which they were written.
The maintainers of the site indicate that they are considering a community effort to update the Jewish Encyclopedia. They do not indicate whether that update will simply be to add more recent scholarship and information about events of the intervening century, or whether there is an intention of revisiting the scholarly apparatus of the old articles to make them more accessible to the modern reader.
- Online version of the 1901–1906 Jewish Encyclopedia; their approach to transliteration of Hebrew and Aramaic is explained at