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!

edwardrpool

This plugin is ultra lightweight. It adds a media button above the tinyMCE editor on both the page and post editor, which when pressed copies a shortlink to your clipboard.
The idea behind this plugin is to add short-linking capabilities, without all the bloated code found in other plugins. This plugin does not alter the database, nor is any data saved or collected. It uses the built in WordPresspost/page ID’s in the URL.

ERP ShortLinks

A very lightweight plugin which adds a shortlink button to the post/page editor for quickly sharing on social platforms.

Base64 Shortlinks vs ERP ShortLinks