Britannica Internet Guide - http://www.britannica.com/
Britannica's magazine-style look and emphasis on fun and current topics make it friendly to use. It's a good place to start when looking for popular topics. Britannica also allows keyword search through its topics.
BUBL Link - http://bubl.ac.uk/
This excellent British-based subject directory allows you to either search or browse through its index. It specializes in being the Web's best Dewey Decimal System catalog, but you can also hunt by such things as subject, author or abstract. Focused mainly on higher education topics.
INFOMINE Scholarly Internet Resource Collections - http://infomine.ucr.edu/
Compiled by the University of California at Irvine, Infomine is another directory specializing in pointing to collections of information on the web, or metasites. It does not summarize or review sites; it does allow keyword search. Academically-oriented topics.
Scout Report - http://scout.cs.wisc.edu/
Internet resources, as chosen by the editorial staff of the Scout Report, cataloged and organized for efficient browsing and searching. This directory is an archive of sites reported in The Scout Report, a current awareness newsletter listing noteworthy web resources for educators and researchers.
Yahoo! - http://www.yahoo.com
With over half a million in its index, this grand-daddy of search tools may be the largest subject directory on the web, meaning that it covers more and is less selective than other directories. It has searching both by subject and by keyword.