LDAP

http://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol

http://www.tldp.org/HOWTO/LDAP-HOWTO/authentication.html

http://oss.segetech.com/linux-ldap-configuration.html

http://www.pubcookie.org/faq.html for SSO

http://directory.apache.org/community%26resources/ldap-renaissance.html is it attractive when compared to openLDAP?

Older howto http://linuxwiki.riverworth.com/index.php?title=LDAP_Authentication but fairly comprehensive

http://www.bekatul.info/node/24 includes nsswitch

Ed Dumbil's writeup http://times.usefulinc.com/2005/09/25-ldap