The default WordPress “shortlink” URLs look like this: <code>http://www.example.com/blog/?p=123</code>, where “<code>123</code>” is actually a 7+ digit decimal number. This plugin shrinks your shortlinks by encoding that number into only 4 characters, and using the abbreviated URL prefix of your choice.
On my site, the default shortlinks are 35 bytes long, even though I have a very short domain name. This plugin shrinks them to 21 total bytes, which is comparable to most public URL-shortener services, and better than many.
E.g., this: <code>http://www.jwz.org/blog/?p=13240780</code> 
becomes: <code>http://jwz.org/b/ygnM</code>
This doesn’t affect your (long) permalinks: those can still be in whatever format you like.

Base64 Shortlinks

This plugin makes your shortlinks shorter!

Kostas Vrouvas

Get the classic “Get shortlink” from WordPress 3.7. Developed to make it easier for people at <a href="https://mentorinculture.com/" rel="nofollow ugc">Mentor</a> to get shorlinks and open sourcing it.
To-do:
<ol>
<li>Connect with Bit.ly API</li>
<li>Options menu to add Bit.ly API keys</li>
</ol>

Get Shortlinks

Get the classic "Get shortlink" from WordPress 3.7. Developed to make it easier for people at Mentor to get shorlinks and open sourcing it.

Base64 Shortlinks vs Get Shortlinks