Automattic

John Godley

Donncha O Caoimh (a11n)

Eoin Gallagher

Lenny

Future upgrades to Sharedaddy plugin will only be available in <a href="http://jetpack.me/" rel="nofollow ugc">Jetpack</a>.
Jetpack connects your blog to the WordPress.com cloud, <a href="http://jetpack.me/support/" rel="nofollow ugc">enabling awesome features</a>.

Sharedaddy

Future upgrades to Sharedaddy plugin will only be available in Jetpack.

demixpress

This plugin makes it even easier and better to display <a href="http://addthis.com" rel="nofollow ugc">AddThis</a> tools for your WordPress site, including share buttons, follow buttons and recommended contents.
To make this plugin to work:
<ol>
<li>You must have an AddThis account.</li>
<li>You CAN NOT use other AddThis plugins.</li>
</ol>
About Social Sharing Optimization
This plugin doesn’t do anything to optimize the social sharing results, if you need this, try a related plugin, such as:
<ul>
<li><a href="https://wordpress.org/plugins/wpsso/" rel="ugc">WordPress Social Sharing Optimization</a></li>
<li><a href="https://wordpress.org/plugins/wonderm00ns-simple-facebook-open-graph-tags/" rel="ugc">Facebook Open Graph, Google+ and Twitter Card Tags</a></li>
</ul>

(dp) AddThis

Makes it even easier and better to display AddThis tools (share buttons, follow buttons and recommended contents) on your WordPress site.

Sharedaddy vs (dp) AddThis