If you're into chatting and use the Adium client, the AdiumBook might be of your interest. This smart, single windowed application lets you sync your contacts between Adium and your Address Book.
AdiumBook is really easy to use and that is mostly due to the simple interface. On one side you see all your Adium contacts, on the other hand your Address Book entries. Sync them simply by dragging and dropping. AdiumBook also features a search box at the top right of each section, by which you can quickly look for contacts.
To make sure that both applications are well synced you can generate reports, showing which contacts are in one list and not in the other, those which don't have a picture, or group entries by IM service.
AdiumBook isn't the most advanced application, and doesn't even features a preferences pane. Once you have all your contacts synced you probably won't need using it anymore.
Despite its limitations, AdiumBook is a simple little program to easily sync contacts betwen Adium and your Address Book.