ABC is a multi-user address book. It uses PHP and MySQL to present a very flexible address database to its users.
It also makes use of AJAX, which means that page reloads only happen on login/logout - the rest of the operations is handled asynchronously in the background. It's well usable, even over slow connections.
You can also install ABC on your desktop PC and create special bookmarks like http://localhost/abc/?login=username so you don't have to login if your request comes from the same computer on which the web server sits. Of course this behaviour is configurable.
There's a playground for all who would like to try ABC: http://abc-test.dnsalias.org/. Correct usernames are "test", "test1" and "test2", each account has the username as password. Go there and play with it!
You can get the Address Book Continued package here: sourceforge.net file release system
You can obtain the source code from https://github.com/mathisdt/abc.
Important: You have to create an initial user (before you can use ABC) by executing something like
INSERT INTO user (username, password) VALUES ('yourusername', 'yourpassword')
with your preferred database administration tool, e.g. MySQLAdmin!