The best-known catalog, and the first, is The WWW Virtual Library (URL is http://www.w3.org/hypertext/DataSources/bySubject/Overview.html ), maintained by CERN. The Virtual Library is a good place to find resources on a particular subject, and has separate maintainers for many subject areas.
Yahoo (URL is <URL:http://www.yahoo.com/> ) is probably the most complete hierarchical, topical index of web sites, and also features a sophisticated search facility.