There have been discussions about sharing Address Book contacts through LDAP, and there have been many suggestions such as running OpenLDAP, using .Mac, or running dedicated apps on your Mac. In this day and age of Google Apps, I would think that someone (Google?) would have come up with a shared LDAP server that is free or low cost. Yes, there's Plaxo, but it's not LDAP -- standards exist for a reason, and Apple has included LDAP client capabilities in Address Book for a while.
Then I stumbled on FreeLDAP.org. I think this is the only free LDAP server available on the Internet that could be used by multiple users accessing the same database. The setup instructions on the site do not work with the Mac's Address Book. However, while browsing through their forums, I found this setup which works for me on my 10.4 system.
First you have to signup for a new free account at freeLDAP.org, and remember the username and password. Then, in Address Book, go into Preferences » LDAP and choose New. Enter the following information:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080426172301710