http://www.addthis.com/ (http://www.addthis.com/)
I haven't seen this mentioned here yet, it's kind of a handy thing to have on your site. It adds a row of buttons that make it easy for people to share it on Facebook and Twitter and the like
Around here we seem to be in favor of the Sociable plugin. Personally, I don't like the AddThis toolbar because if I hover over it, even by accident, my browser freezes up for like 15 seconds until the expanded menu appears. At least when it's the default style.
I like AddThis, but I may opt for an alternative soon, as it seems to pull up its popup even when I don't hover over the button sometimes. It also causes a little gap to appear at the top of whatever page it's on. It took forever to find that out, and it's annoying as hell.