Plaxo for Mac

From Plaxo:

Plaxo for Mac synchronizes your Mac OS X Address Book with your Plaxo Universal Address Book. Wherever you install Plaxo for Mac, 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.

On Mac OS 10.4.x and newer, Plaxo for Mac also offers the ability to sync your iCal calendars to Plaxo Online (and on to any other Plaxo sync points, e.g. Google Calendars, Outlook, etc) For iPhone users, this is a real boon as your contacts and calendars are now kept in sync on your phone, desktop and the web (and multiple Macs and PCs should you have more than one computer)

Plaxo keeps people connected by solving the common and frustrating problem of out-of-date contact information. Founded in July 2001, Plaxo provides a free service that securely updates and maintains the information in your address book. Plaxo is the universal digital assistant; available to you wherever and whenever you need it to keep you organized, on top of your life and in touch with those you care about.