When you use a search engine, you are not actually searching the Web itself but rather the search engine's index of the sites it has selected. Results will differ according to how frequently the index is updated, how large the index is, how the sites are retrieved and how the search engine searches its own index. Some of the most frequently used search engines and directories are listed below. You will want to explore these and some others to find the ones that are most useful to you. They act as intelligent middle men between the web sites and the search engines and can be a good starting place for some searches.
Simple searches work best.
Organizes lots of search engines by category and provides links to their sites with brief annotations. An annotated guide to 13 search engines. Advanced searching provides nested Boolean operations. Boolean operations, thesaurus terms, and narrowed terms can be added after retrieval of initial search results. Provides Web links and canned PubMed searches.