brehash

This plugin adds the official <a href="http://blog.twitter.com/2010/08/pushing-our-tweet-button.html" rel="nofollow ugc">Tweet Button</a> 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.

rsh-Tweet

Adds the official Tweet Button from Twitter.com.

dantaylorseo

This plugin adds the all new Twitter.com button to your website.
<ul>
<li>Choose whether to use RT or via</li>
<li>Choose position of button</li>
<li>Choose style of button</li>
</ul>
The easiest to install Official Twitter.com button!

New Twitter Button

Adds the all new Twitter.com button to your website.

rsh-Tweet vs New Twitter Button