Ready Reference Quotations

Bartlett's Familiar Quotations: Published in 1901.
Dictionary of Scientific Quotations: Notable quotes from scientific disciplines.
Famous-Quotations.Com: A large index of quotations searchable by both author, keywords, and category.
Quotation Resources and Links Research Service: Quotation resources arranged by topic.
Quotations from the Daily Miscellany: Extensive list of quotation pages, organized by subject area.
Quoteland.com: Searchable database of quotations by Topic, Literary Quotations, Humorous Quotations, Random Quotations, How to Identify a Quote, Discussion Groups ["Who Said It?" and "I Need A Quote!"], a FAQ, and Internet Resources. There is also a Reference Library with a short list of audio samples from historical speeches, TV commercials, TV and movie scripts, and music.
Simpson's Contemporary Quotations: 1988, provides over 10,000 quotations from over 4,000 sources, organized into 25 categories and 60 sections. Full, concise information is given for each quotation, including source, date, and occasion.
Yahoo: Reference: Quotations: List of links to quotation resources.