I really like Tweetmeme. It works like this - a button on your blog encourages visitors to tweet your posts, it displays how many people do this and that has the effect of encouraging more people to do the same.
While all this tweeting is going on Tweetmeme monitors which are the popular blog posts and once a certain threshold has been reached the @tweetmeme Twitter profile tweets out a general link to the popular post. It's a good way of surfacing interesting posts.
Over the last few hours, though, Tweetmeme seems to have been successfully gamed by annoying follower spam.