Jo Sprague

Quotable adds buttons to quotes and text selection that make it quick and easy for your readers to share quotes from your website.
Simply install the plugin, there is no setup required. Icons that match your site’s theme will be added to quotes and appear in a contextual menu when readers select text on your website, allowing them to quickly and easily share the quotes to Twitter.
This plugin adds a page link, hashtags and mentions to shared quotes. WordPress tags are used as hashtags and, if you use Yoast’s WordPress SEO plugin, the post author’s and site’s Twitter accounts are added as mentions to Tweets.
<h3>Sponsorship</h3>
I am software engineer and I love to create free and open source software for nomads, bloggers and language learners. In order to afford to spend time creating, maintaining and updating my free software, I rely on financial support from donors. In return, I provide my donors with access to exclusive updates, support and features. If you find this plugin useful, please consider <a href="https://www.patreon.com/localjo" rel="nofollow ugc">supporting me for just $1/month on Patreon</a>.

Quotable

Adds buttons to quotes and text selection that make it quick and easy for your readers to share quotes from your website.

nektobit

A lightweight (8Kb minified, 2.9Kb gzipped) script to let users share their text selections to social networks, in desktop browsers. (Yes, like Medium). 
Based on Sharect.js
<ul>
<li>Supports adding custom icons (for now using php snippet)</li>
<li>You can also control the color of the tooltip widget (also using php code)</li>
</ul>
<a href="https://nektobit.github.io/WP-Sharect/#documentation" rel="nofollow ugc">Documentation</a>
Millstones
The plugin is under active development, I welcome any feedback.
<ul>
<li>Add widget color picker</li>
<li>Adding social media icons from the admin interface</li>
<li>Optional meta tags</li>
</ul>

Sharect WP

The plugin allows you to highlight text on your website and share it on social networks

Quotable vs Sharect WP