ruudkok

<p>With this plugin you can add the new Tweet button from Twitter to every post, and every page. The plugin is fully customizable:</p>
<ul>
<li>Choose where to display the button (top,bottom,left,right), up to 2 times each post/page</li>
<li>Choose when to display the button (posts, pages, both or everywhere)</li>
<li>Select the type of button you like (horizontal count, vertical count, or no count)</li>
<li>Set the margins between the Twitter Button and surrounding content</li>
<li>Select a twitter account (if any) which you want to be mentioned in each tweet (e.g. via @ruudkok)</li>
</ul>


WP Twitter Button

This plugin allows you to add the new Twitter button(s) to your blog. You can choose the type of button and set where to display the buttons.

Pete Mall

<p>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.</p>


WP Tweet

Adds the official Tweet Button from Twitter.

WP Twitter Button vs WP Tweet