Those Dark Hiding Places: The Invisible Web Revealed. This is a good article by Robert J. Lackie, Associate Professor-Librarian, Rider University. Although it has a 2001 copyright on it, the material is clearly being updated still.
From the site:
"If only I had known!" was the bitter cry of the searcher who relied just on search engines to search the Web. Although many popular search engines boast about their ability to index information on the Web, more of it (dynamically-generated pages, certain file formats, and information held within numerous databases) has become invisible to their searching spiders. Much of the Web is hiding information from us, but we can access this hidden content! Learn how you can reveal the secrets of these dark, hiding places.