The Apple Developer Connection Publishes "Writing Open Directory Plug-ins:"

Open Directory is Apple technology that lets Macs participate on a network and access network resources, along with Windows and UNIX machines. For example, with Open Directory, an application on Mac OS X can allow users to update their personal information in a company-wide directory of employee data, even if the database is running on Linux or Windows. This article describes the implementation of an Open Directory plug-in, which responds to requests for information about resources. Read this article to learn how to write Open Directory plug-ins that take advantage of this capability.