If you use Facebook, you might be interested in syncing your friends' profile images and birthdays into your OS X address book. After trying a number of programs, none met my needs. So I created AddressBookSync, a free Facebook/OS X tool which matches Facebook friends to contacts in Address Book, and is able to sync their profile picture and birthday information. This utility is probably most useful for people who sync their phones (such as an iPhone) with their OS X Address Book for contact information. This way, they can keep their contacts' pictures updated with their latest and greatest Facebook profile picture, which will show up when they call.
AddressBookSync does not and will not support the synchronization of phone numbers or email addresses as Facebook does not allow this by their Terms of Service.
[robg adds: I haven't tested this utility. Note that this hint was submitted by the program's author, but given that it performs a useful service and seems to be completely free of charge (and any advertising in the app or the linked site), I felt it worth sharing.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081121121700806