Barry Carlyon

This plugin adds extra bits an pieces to <a href="https://wordpress.org/extend/plugins/jetpack/" rel="ugc">WordPress Jetpack</a>
Previous called “Jetpack Extras” renamed at request of Jetpack Team to “Custom Tweaks for Jetpack”
Which includes the following additions:
<ul>
<li>
Ability to control button placement, above, below, or both of the post content, with separate options for the archive page and content display page
</li>
<li>
Twitter Button added Related (username/optional description format)
</li>
<li>Adds the ability to make the Twitter button share the WP.me url, if that JetPack module in use, <a href="http://skipsloan.com/?p=175" rel="nofollow ugc">As suggested by SkipTweets on Twitter</a></li>
</ul>
Currently removed is:
<ul>
<li>Ability to turn on/off the <a href="https://dev.twitter.com/docs/tweet-button#optout" rel="nofollow ugc">DNT Twitter</a> button mode – it is difficult to add without editing chunks of the core</li>
</ul>

Custom Tweaks for Jetpack by BarryCarlyon

Extras for WordPress Jetpack. Tested up to Jetpack 3.4.1

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"

Custom Tweaks for Jetpack by BarryCarlyon vs Jetpack Twitter Via