Information Please Includes almanacs, dictionaries, and encyclopedias.
Internet Public Library Maintains a collection of online ready reference works, and evaluates and categorizes resources on the Internet.
Internet Scout Report: Archives Internet sites cataloged and organized for efficient browsing and searching.
Librarians' Index to the Internet Searchable, annotated subject directory of more than 7,000 Internet resources.
Martindale's Reference Desk Detailed subject guide to the Internet.
Metaplus Index to reference sources.
Reference Desk A wealth of reference information on numerous subjects.
The WWW Virtual Library Index to information on the Internet organized by subject.

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Google: Reference
Yahoo!: Reference
Librarians Index to the Internet: Reference
BUBL: Reference
DMOZ: Reference