The Invisible Web

The majority of information available online CANNOT be found via the major search engines! A whole host of databases, web pages, websites, and image files are not listed because either they cannot be spidered or because search engines choose not to include them in their indexes.

Unfortunately it means many invaluable resources are difficult to locate e.g
Subject Guides - Web resources dedicated to cataloging hyperlinks to web pages about a particular subject.
Reference Works - Generally self contained documents that may not contain hyperlinks to other resources. For example dictionaries, encyclopedia and maps.
Specialized Databases - Self searchable indexes that catalog information such as patents, medical journal article citations, company financial data an so on.

Invisible Web Resources lii.org - Librarians' Index to the Internet Help    Advanced Search INFOMINE Scholarly Internet Resource Collections Advanced Search Internet Public Library University of Michigan School of Information Pathfinders - Topic Research Guides