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"

Jeremy Herve

Extends the Jetpack plugin and allows you to add sharing buttons anywhere inside your posts thanks to the [jpshare] shortcode
Important: for this plugin to work, you must activate <a href="https://wordpress.org/extend/plugins/jetpack/" rel="ugc">Jetpack</a> first, and activate the sharing module.
This plugin is a work in progress. You can report issues <a href="https://wordpress.org/support/plugin/jetpack-sharing-butttons-shortcode" rel="ugc">here</a>, or submit a pull request <a href="https://github.com/jeherve/jp-sd-shortcode" rel="nofollow ugc">on GitHub</a>.

Sharing buttons shortcode for Jetpack

Extends the Jetpack plugin and allows you to add sharing buttons anywhere inside your posts thanks to the [jpshare] shortcode

Jetpack Twitter Via vs Sharing buttons shortcode for Jetpack