Others are generic, all-purpose tools. This is a selective listing. Similar to following the references found in a key journal article, this can yield nuggets. Given the size of the Internet, however, this is a marginal approach for serious inquiries. Site selection is done by humans.
Many directories are also searchable.
All search engines vary as to how they build and maintain their databases, resulting in substantial variability in search results. Therefore, comprehensiveness improves by the use of multiple search engines. Again, each works in a unique manner and searches on a different combination of resources. The Web resources listed here are designed for health professionals, although health consumers may also find them useful. All provide free access.