This plugin adds the official
Tweet Button from Twitter to your Pages, Posts or Home Page. It lets your users share links directly from the page they're on. When they click on the Tweet Button, a Tweet box will appear pre-populated with a shortened link that points to the item that they're sharing.
This plugin is very customizable via 'rsh-Tweet Button' under the 'Settings' menu in WordPress Admin. Change the look and feel, pozition and where to seen of the button.