Subject Headings:

Civilization, Classical – Encyclopedias
See also: Classical Dictionaries

Civilization of the ancient Mediterranean : Greece and Rome, ed. M. Grant and R. Kitzinger.  New York : Scribner's, c1988.
Perkins -- DE59 .C55 1988
A good introductory 3-volume encyclopedia, with essays on a wide variety of topics from classical technology to politics to religion.  Essays are by recognized scholars and include bibliographic suggestions. 

Pauly's Real-Encyclopadie der classischen Altertumswissenschaft, ed. A. F. von Pauly and G. Wissowa.  Stuttgart : J. B. Metzler, 1894-1963.
Perkins Reference -- DE5 .P33 1894
Commonly known as "Pauly-Wissowa", or just "Pauly", this encyclopedia has long detailed essays, including extensive references, about a dazzling variety of specific topics in classical antiquity.  This is an older, but still invaluable, resource, and often handily summarizes the extensive 19th century German scholarship on a topic.  Articles are primarily in German, but also in other European languages.

Der Kleine Pauly: Lexikon der Antike, ed. K. Ziegler and W. Sontheimer. Stuttgart, A. Druckenmüller, 1964-75.
Perkins Reference -- DE5 .K5 1964. 
An update and abbreviation of the original Pauly-Wissowa, again mostly in German.

Der neue Pauly: Enzyklopadie der Antike, ed. H. Cancik and H. Schneider.  Stuttgart : J.B. Metzler, 1996-2003.
Perkins Reference -- DE5 .N48 1996. 

Brill's New Pauly : encyclopedia of the ancient world: Antiquity, ed. H. Cancik and H. Schneider.  Leiden ; Boston : Brill, 2002-
Perkins Reference -- DE5 .N4813 2002. 
An update of the original Pauly-Wissowa, translated into English and currently in progress.

New Pauly Online
Updated version and translation of Pauly's Real-Encyclopädie der classischen. The encyclopedia itself includes the original German work and its English translation.