Konstantin Obenland

This plugin needs Jetpack by WordPress.com to be activated and its Sharing module to be enabled. 
Enter the desired Twitter usernamen in Settings -> Sharing. It will then be appended, every time a visitor shares your content on Twitter
See the <a href="https://dev.twitter.com/docs/tweet-button" title="Tweet Button" rel="nofollow ugc">Tweet Button documentation</a> for details.
<h4>Translations</h4>
I will be more than happy to update the plugin with new locales, as soon as I receive them! 
Currently available in:
<ul>
<li>English</li>
</ul>

Jetpack Twitter Via

Adds 'via @username' to the Tweet Button provided in "Jetpack by WordPress.com"

Mark Jaquith

The <a href="https://wordpress.org/extend/plugins/jetpack/" rel="ugc">Jetpack</a> plugin by Automattic does something sneaky: it auto-activates new Jetpack modules. Instead of allowing you to opt in to new modules, Jetpack cheerfully activates them without your permission. These modules can change how your site works and looks.
Manual Control puts a stop to that unwanted behavior. It prevents auto-activation of new Jetpack modules.
Metal texture in the plugin header image by <a href="http://urbandirty.com/gallery/photo/2568483179-m" rel="nofollow ugc">Urban Dirty</a>.

Manual Control for Jetpack

Jetpack auto-activates new modules, without asking your permission. This plugin stops that.

Jetpack Twitter Via vs Manual Control for Jetpack