LDAP: Lightweight Directory Access Protocol
Tiki offers LDAP Authentication, see doc:LDAP authentication
Here are some good articles I found useful to get me up to speed on LDAP. - cbarbry
Top Five Open Source Packages for System Administrators by Æleen Frisch — The countdown continues this week with number four, LDAP. Æleen Frisch, author of Essential System Administration, 3rd Edition, offers the second installment in a five-part series on the most useful and widely applicable open source administrative tools.
An Introduction to LDAP by Luke A. Kanies — Luke A. Kanies introduces LDAP and explains why it is an important tool for network administrators.
Getting Started with LDAP by Luke A. Kanies — Luke A. Kanies shows you how to set up a basic LDAP directory to store Unix user accounts, along with a script to pull those accounts to a Unix system.
Building an Address Book with OpenLDAP by Dustin Puryear — LDAP's most sensible example is, perhaps, managing contacts throughout a company. There aren't many good examples of this, however. Dustin Puryear wants to change that. In this article, he demonstrates how to build and populate a company-wide LDAP address book.