I am not downloadingthis yet, but it could be promising.
– Integrates with OS X Address Book
– Syncs with all your other clients
– Lightweight and easy to use
The Plaxo Toolbar for Mac synchronizes your Mac OS X Address Book with your Plaxo Universal Address Book. Wherever you install a Plaxo toolbar, your address book will be consistent and stay up-to-date automatically. You can also access your address book on the web through Plaxo Online.