The Apple directory services architecture, which can access authoritative information about users and network resources from directory domains that use LDAP, NetInfo, or Active Directory protocols; BSD configuration files; and network services.

a directory (folder) on your webserver that does not contain a default html page

a volunteer-built guide to the web

The Open Directory Project (ODP), also known as DMoz (for Directory.Mozilla, the domain name of ODP), is a multilingual open content directory of World Wide Web links owned by Time Warner that is constructed and maintained by a community of volunteer editors.

The Open Directory uses volunteer editors to catalog the web. Formerly known as NewHoo, it was launched in June 1998. It was acquired by AOL Time Warner-owned Netscape in November 1998, and the company pledged that anyone would be able to use information from the directory through an open license arrangement.

The larges, but perhaps least used global Internet Directory.  Its database is used by virtually all the other directories as a source of website urls.  It has been said that unless a site is registered with Open, Google will not register it on its own pages

A popular search directory owned by Netscape and syndicated by Google and many other Web sites.

Massive directory of websites reviewed by humans at http://www.dmoz.org. Results also appear on many search engines including the all important Google.