Some directories include an internal search engine that searches only these preferred sites, bringing back hits that tend to be highly relevant, and often more academic in nature. Perhaps the most rigorously reviewed directory, Argus specializes in websites of academic import on a variety of subjects. Does not include an internal search engine. This criteria is given up front at the Argus site. Inlcudes its own search engine.
Every site is accompanied by a review.
A periodic report of librarian-reviewed sites in the areas of business and economics, social science, and science and engineering. Includes an internal search engine. Also a subject list arranged by Library of Congress classification. However, the search engine is also good. Type in your topic, and then link directly to a site, or click on the subject category that will bring up a predetermined set of subject-related entries.