This tutorial explores a variety of search tools to help you gain skills in conducting research on the Internet. More recent innovators are covered in Second Generation Searching on the Web.
Directories are a good place to find invisible Web content.
In addition, there are collections devoted to listing database sites. The Internet is a self-publishing medium. Your visits to subject directories and search engines will yield files with a wide range of quality from a variety of sources. Be sure to evaluate everything you encounter. For more information, see Evaluating Internet Resources.