ruudkok

With this plugin you can add the new Tweet button from Twitter to every post, and every page. The plugin is fully customizable:
<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.

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.

WP Twitter Button vs WP-Twitter Retweet Button