H. Peter Pfeufer

Twitter has provided a twitter and retweet button. This Plugin adds the official twitter button to your blog articles, pages, archives, categories and rss.
Features
<ul>
<li>Easy install</li>
<li>Panel for easy configuration</li>
<li>Config Button Style</li>
<li>Config Language of Button</li>
<li>Config Your Twitter Account</li>
<li>Config Tweet text</li>
<li>Config display locations (entries, pages, home page, archive, category)</li>
<li>Config position in content (before, after, both, manual choice)</li>
</ul>
Note
If you choose the position as “manual choice” you have to edit your template-file and add the following code at a position of your choice. 
 <?php if(function_exists(‘wp_twitterbutton’)) { wp_twitterbutton(); } ?>
Translations
<ul>
<li>English</li>
<li>German</li>
<li>French (by: <a href="http://www.frankoli.de/blog/" rel="nofollow ugc">Frank</a>)</li>
<li>Dutch (by: <a href="http://wpwebshop.com/" rel="nofollow ugc">Rene</a>)</li>
<li>Czech (by: <a href="http://blog.michalbernat.cz/" rel="nofollow ugc">Michal Bernat</a>)</li>
<li>Hebrew (by: <a href="http://www.sagive.co.il/" rel="nofollow ugc">Sagive</a>)</li>
<li>Hindi (by: <a href="http://outshinesolutions.com/" rel="nofollow ugc">Love Chandel</a>)</li>
</ul>

WP-Twitter Retweet Button

This plugin will install the official Twitter Button provided by Twitter to your Wordpress-Blog.

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.

WP-Twitter Retweet Button vs WP Tweet