Philip John

This plugins adds a visible link to the end of every post.
Best used in conjunction with the <a href="https://wordpress.org/extend/plugins/googl-shortlinks/" rel="ugc">Goo.gl Shortlinks</a> or <a href="https://wordpress.org/extend/plugins/bitly-shortlinks/" rel="ugc">Bit.ly Shortlinks</a> plugins.
Feature requests welcomed with open arms!

Add Shortlink to Posts

Adds a link to the shortlink for each post below the content.

Boone Gorges

<a href="http://yourls.org/" title="Your Own URL Shortener" rel="nofollow ugc">YOURLS</a> is a free URL shortener service you can run on your webhost to have your own personal Bitly.
This plugin is a bridge between <a href="http://yourls.org/" title="Your Own URL Shortener" rel="nofollow ugc">YOURLS</a> and your blog: when you’ll submit a new post or page, your blog will tap into YOURLS to generate a short URL for it.
This plugin also features BuddyPress support. Short URLs can be generated automatically for each member and each group, and are customizable if permitted by the BuddyPress administrator.
Requires PHP 5.
Note: this plugin should still work but is no longer maintained. See http://yourls.org/#More#wordpress

YOURLS: WordPress to Twitter

Use YOURLS (a free GPL URL shortener service) to create short URLs of your posts, or for your BuddyPress groups and users

Add Shortlink to Posts vs YOURLS: WordPress to Twitter