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.

zackproser

The Simple Twitter Follow Me button allows you to add a small or large Twitter follow button to the end of every post so you can grow your Twitter following with your blog readers and let your audience easily share your content on social networks.
Once installed and activated, go to Settings -> Reading and scroll down to the Simple Twitter Follow Button settings.
Here you can enter your Twitter username and choose whether the button should be small or large.

Simple Twitter Follow Me Button

This plugin is the easiest way to add an official Twitter follow button at the end of every post so your readers can follow you on Twitter.

WP Twitter Button vs Simple Twitter Follow Me Button