There are a number of directory services that are used widely. Two of the most important ones are LDAP, which is used primarily for e-mail addresses, and Netware Directory Service (NDS), which is used on Novell Netware networks. Virtually all directory services are based on the X.500 ITU standard, although the standard is so large and complex that no vendor complies with it fully.

Directory Services Reading Room
Collection of primers and articles about NDS, DCE, NIS/NIS+, and LDAP.

What is a directory service?
Very complete description of directory service, with sections on the X.500 standard, implementations of computer-supported directories, and the structure of the stored information.

Yahoo!'s directory services page
Yahoo!'s directory of directory services.