CollegeDegree.com has just produced a nice resource, 99 Resources to Research & Mine the Invisible Web. It’s a list of deep web search engines, databases, catalogs, directories, and social media sites. At the bottom of the list are references to a number of guides about the deep web. The deep web consists of content that usually lives in a database, is accessed by humans through web forms, by federated search engines with specialized knowledge of deep web sources, and is not easily accessible to Google and other web crawlers.
I’ve added this document to the resources page.
This “99 resources” guide is definitely worth a bookmark.