Pete Mall

This plugins enables you to add the official <a href="http://blog.twitter.com/2010/08/pushing-our-tweet-button.html" rel="nofollow ugc">Tweet Button</a> from Twitter. 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.

WP Tweet

Adds the official Tweet Button from Twitter.

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.

WP Tweet vs rsh-Tweet