Got to know about it recently and signed up.
Their Alexa and Technorati ranks look high (as per the graphs on their site).
Rohit Bhargava, VP of Interactive Marketing at Ogilvy PR Worldwide, recommends AddThis (again, according to the AddThis site).
I signed up. They have cool widgets that you can put on:
1) a web page (the bookmark widget)
2) a blog (the feed widget)
Check out the bottom of the home page of my site - DancingBison.com - for an example of the bookmark widget.