AddThis is a new widget service that was launched in October 2006. With so many bookmarking services popping up these days, something like this was imminent. By Creating an account on AddThis blog and site owners could generate a simple, free button that could be placed at the end of posts (see sample button at the end of this post). Readers can then bookmark the post on Digg, StumbleUpon, Google Bookmarks, Technorati, Reddit and the likes with a single click on the button. The service also provides with statistics including content that your readers bookmarked the most, over various periods of time.
AddThis claims that they have served 100+ million buttons to-date, with 2 million buttons being served daily.
From a usability perspective AddThis removes the clutter of multiple bookmarking icons at the end of the posts and replaces them with a clean single button. However this creates indirection and in order to bookmark something you will have to click on the AddThis button and then select the bookmarking service on the next page. Clearly this is an execution speed vs less clutter scenario. Apart from this, the service is easy to use and is gathering steam.
Also see Techcrunch´s take on this.