The YIVO Encyclopedia of Jews in Eastern Europe is online in full:
The only resource of its kind, this encyclopedia provides the most complete picture of the history and culture of Jews in Eastern Europe from the beginnings of their settlement in the region to the present. This Web site makes accurate, reliable, scholarly information about East European Jewish life accessible to everyone.
The first thing I looked up was “Kiev,” and I found not only a full article, with history, photos, and bibliography, but a map showing the location of not only the synagogues but the Jewish Theater, the Jewish Market, the Jewish Bathhouse, the Jewish Gate (from the 11th century), the Zionists Club, the houses where Ilya Ehrenburg, Sholem Aleichem, and other well-known Jews lived, the Continental Hotel (where Mandelstam, Ehrenburg, and Babel lived at various times)… well, you get the idea. It’s a treasurehouse, and I look forward to exploring it at length. Via Dumneazu (“Ethnomusicological Eating East of Everywhere”).