http://www.w3.org/hypertext/DataSources/bySubject/Overview.html
This site links the user to indexes for more than one hundred specific subject categories. The home page lists the hierarchies in alphabetical order, but the user can also see a these categories grouped by subtree, by Library of Congress classification standards, by popularity, or by type of server (WWW, gopher, etc.). Most subdirectories are not actually on this server. Instead, they are maintained on outside servers by different volunteers across the world.