|Remember that subject directories are built and maintained by people, and are arranged by subject areas. They are sometimes annotated, and are a good source of information.
About.com: http://about.com/ - a commercial site, About.com is a good directory with many contributors. However, you will get pop-up ads, which is distracting
BUBL Link: http://bubl.ac.uk/link/index.html - a European directory, and a very good quality site. Worth looking at for a different approach!
DiscoverySchool.com: http://school.discovery.com/schrockguide/yp/iypdir.html -a list of some good subject directories
Infomine: http://infomine.ucr.edu/ - Over 30 university and college librarians have contributed to building INFOMINE. Hosted by University of California at Riverside.
Infopeople: http://www.infopeople.org/search/chart.html - scroll down the page for the part on subject directories
Internet Public Library: http://www.ipl.org/ - a public service organization and a learning and teaching tool from the University of Michigan School of Information
Internet Scout Project: http://scout.wisc.edu/ - from the University of Wisconsin. A bit less user friendly, but excellent for academic sites
Open Directory: http://dmoz.org/ - one of the largest, most comprehensive human-edited directories on the Web. It is constructed and maintained by a global community of volunteer editors
University at Albany: http://library.albany.edu/internet/subject.html - divided into Academic and Professional Directories, and Commercial Directories and Portals
UC Berkeley: http://www.lib.berkeley.edu/TeachingLib/Guides/Internet/
SubjDirectories.html - has an annotated list of Subject Directories with a Table of Features
WWW Virtual Library: http://vlib.org/ - Unlike commercial catalogs, it is run by a loose confederation of volunteers, who compile pages of key links for particular areas in which they are expert