Bartlett's Familiar Quotations, 10th ed.
Including over 11,000 quotations, the first new edition of John Bartlett's corpus to be published after his death in 1905 keeps most of his original work intact.

Simpsons Contemporary Quotations. 1988.
Contains over 10,000 quotations from 4,000 sources and is organized into 25 categories and 60 sections.

Search a combined database of both contemporary and classic quotations from the these two sources at http://www.bartleby.com/quotations/

Mega-sites
About.com's subject-oriented guide to quotations sources.

The Quotations Page by Michael and Laura Moncur

Yahoo: Reference: Quotations