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!

Erick Hitter

WordPress 3.0 introduced a native shortlinks feature, which builds URLs using query strings. This plugin converts those URLs to ones using pretty permalinks, which makes for a nicer-looking URL and improves shortlinks’ ability to be cached.
Posts and pages are supported by default, while filters make it possible to support other post types.
For example, https://example.com/?p=123 becomes https://example.com/p/123/.

ETH Simple Shortlinks

Convert WordPress's query-based shortlinks to pretty permalinks that are cacheable. https://example.com/?p=123 becomes https://example.com/p/123/.

Base64 Shortlinks vs ETH Simple Shortlinks