If you comment on this blog you will have noticed an image appears next to your comment. This image is a gravatar, and is based on your email address.
According to this page, there used to be a plugin for the OS X Address Book application to retrieve gravatars. Unfortunately the site it links to no longer works.
I therefore wrote my own, which works only on Snow Leopard. The disk image is here. To install it, drag GetGravatar.bundle to Library/Address Book Plug-Ins in your home folder and restart the Address Book application. Click on any email address to be offered the opportunity to download the corresponding gravatar. Be warned, it will replace any image assigned to that contact with their gravatar if they have one.