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!