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.

LinksAlpha

Get Support: support@linksalpha.com
Plugin shows Twitter users who recently tweeted links from your blog. In other words, quick and easy way to get your Twitter community rolling 🙂
Plugin shows Tweets linking your Blog posts. Tweets show up below the comment form on each blog post
Plugin shows Count of Tweets linking to your blog. Count can be configured to show next to “Comment Count” or “Blog Post Date” for each blog post.
Check out our other plugins that you will find extremely useful:
<ul>
<li>Social Discussions: show popularity of your blog posts on Social Networks including Twitter, Facebook, Google Buzz, Yahoo, and bit.ly. https://wordpress.org/extend/plugins/social-discussions/</li>
<li>Social Stats: track your blog activity on social networks – monthly/weekly/daily, and track your popular posts. https://wordpress.org/extend/plugins/social-stats/screenshots/</li>
<li>Network Publisher: auto publish your blog posts to Twitter, Facebook, LinkedIn, Yahoo, Yammer, Identi.ca, and MySpace. https://wordpress.org/extend/plugins/network-publisher/</li>
<li>Buzz Roll: let your users share your blog post on fastest growing social hub – Google Buzz. https://wordpress.org/extend/plugins/buzz-roll/screenshots/</li>
</ul>

WP-Twitter Retweet Button vs Retweeters