It's called the sharebar, and you'll see it hanging on the left on the post pages (but not the front page). It has quite a few options that I'm not using, including digg, reddit, stumbleupon, and others. If there's one you'd like to have, please ask.
Sorry for not adding it sooner; I didn't know anyone wanted it. I don't usually use this kind of thing for sharing links; I use bookmarklets to share because they work everywhere.