Official Twitter "Tweet Button" for blogs and websites.
Prior to Tweet Button, company like TweetMeMe and ReTweet.com use to provide this feature. According to twitter blog TweetMeMe would be helping twitter in developing and resolving issues related to Tweet Button.
This button also shows the count of tweets has been made for the link and also recommend you the people who are associated to that post /link and you could follow them.
You could try out the Tweet Button here; Just click on this button >> Tweet
Check out the Video below to know more about it.