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!

Denis Lam

WordPress generates shortlinks for your posts and pages. By default it uses the <code>?p=</code> with the post ID added to it, but if you have a rather long domain name this isn’t very useful. If you use <a href="http://bit.ly" rel="nofollow ugc">Bit.ly</a>, this plugin will help you replace the shortlink WordPress generates with a proper Bit.ly shortlink.
This plugin uses the latest Bit.ly OAuth 2 draft specification API instead of its deprecated V3 API and enables you to use generic access token to automatically enable Bit.ly shortlinks in your entire multisite network without the need for each site user to have to tinker with any settings or authentication configurations.

Bit.ly Shortlinks Multisite (Uses OAuth 2 API)

This plugin replaces the default WordPress shortlinks with Bit.ly shortlinks for your single site or multisite WordPress network.

Base64 Shortlinks vs Bit.ly Shortlinks Multisite (Uses OAuth 2 API)