AddThis is the #1 bookmarking and sharing button on the Internet.
AddThis (Launched in September 2006) has become the standard button for bookmarking and sharing. The AddThis button spreads your content across the Web by making it easier for your visitors to bookmark and share it with other people, again…and again…and again. This simple yet powerful button is very easy to install and provides valuable Analytics about the bookmarking and sharing activity of your users. AddThis helps your visitors create a buzz for your site and increase its popularity and ranking.
Website: http://www.addthis.com
Web 2.0: Who, What, Why, How 