There are specialised resources with the aim of enabling easier access to the Invisible (or Deep) Web (that part of the web that is not searched by standard search engines). Search engines that index non-html files do this to some extent, as do meta-sites, by drawing attention to the existence of databases and other non-html-based resources.
About.com (Link) provides a useful introduction to searching the Invisible Web. Some of the specialised resources are:
Yippy (Link) combines results from a variety of different sources, filtering out duplicates and sifting the best content that you might not have seen otherwise to the top of the search results.
InfoMine (Link) provides lists of scholarly Internet resources relevant to students, teachers and researchers at the university level. It includes resources such as databases, electronic journals, electronic books, bulletin boards, mailing lists, on-line library catalogues, articles, directories of researchers, and other types of information.
Library Spot (Link) is a collection of databases, online libraries, references, and other good info from the Invisible Web.
OmniMedicalSearch (Link) is a medical search engine. It is divided into four main sections: Search Options, Reference Desk, Conditions and Disease, and Local Directory. The first two of these are perhaps the most useful in the context of this guide. The Search Options include the Web, News, Images, Forums and MedPro which is for those who want to go a little deeper in their research and searches professional peer level resources such as PubMed and Ovid. The Reference Desk allows users to search, Acronyms, Medical Associations, Databases, Dictionary, Health and Medical Forum Directory, Medical Journals and a Medical Image Directory.
SurfWax (Link) allows users to find search results from multiple search engines at the same time.
Virtual Library (Link) is the oldest catalogue of the Web, started by Tim Berners-Lee, the creator of HTML and of the Web itself, in 1991 at CERN in Geneva. Unlike commercial catalogues, it is run by a loose confederation of volunteers, who compile pages of key links for particular areas in which they are expert; even though it isn’t the biggest index of the Web, the VL pages are widely recognised as being among the highest-quality guides to particular sections of the Web.