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.

GeekRMX

This plugin allows you to easily add Twitter @Anywhere to your blog, enabling the @Anywhere features:
<ul>
<li>Auto-linkification of @usernames</li>
<li>Hovercards</li>
<li>Follow button</li>
<li>Tweet Boxes</li>
<li>Retweet button</li>
</ul>
You can enable and configure each function from the plugin settings page:
<ul>
<li>Customize the label, content and dimensions of your Tweet Boxes.</li>
<li>Display the post title and short URL into the Tweet Box.</li>
<li>Shorten your URLs with bit.ly or is.gd.</li>
<li>Insert custom Tweet Boxes by using shortcodes.</li>
<li>Clicking the Retweet button will launch a special Tweet Box with Lightbox effect (<a href="https://wordpress.org/extend/plugins/twitter-anywhere-plus/screenshots/" rel="ugc">screenshot</a>).</li>
<li>Choose from two Twitter birds to be displayed above the Retweet Box 🙂</li>
</ul>
Languages:
<ul>
<li>English (en_US)</li>
<li>Spanish (es_ES)</li>
<li>French (fr_CA)</li>
<li>Belorussian (be_BY)</li>
</ul>
In order to use @Anywhere, you must first register your blog for a free API key with Twitter. You can do so at the following URL: http://dev.twitter.com/anywhere/apps/new (<a href="https://wordpress.org/extend/plugins/twitter-anywhere-plus/faq/" rel="ugc">FAQ</a>)
<a href="http://www.ngeeks.com" rel="nofollow ugc">nGeeks.com</a>
<h3>Translations</h3>
<ul>
<li>English (en_US) [default]</li>
<li>Spanish (es_ES)</li>
<li>French (fr_CA) – <a href="http://siliticx.ca" rel="nofollow ugc">Charles Desaulniers</a></li>
<li>Belorussian (be_BY) – <a href="http://pc.de" rel="nofollow ugc">Marcis G.</a></li>
</ul>
You can help translating the plugin into your language. 
Use the <code>tap-default.po</code> file included with the plugin.

Twitter @Anywhere Plus

This plugin allows you to easily add Twitter @Anywhere to your blog, enabling the @Anywhere features.

WP-Twitter Retweet Button vs Twitter @Anywhere Plus