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!

Mono WP

Prime Affiliate Links
Prime Affiliate Links is an easy-to-use plugin that helps you sky-rocket your affiliate marketing business by providing a link management solution that allows you to shorten, cloak, and manage your links easily and efficiently.
Prime Affiliate Links enables you to shorten and prettify your affiliate links using your own domain name as opposed to using some random link shortening service (like tiny.url or bit.ly).
Prime Affiliate Links also comes with advanced link stats to help you understand how your affiliate links are performing and tons of other useful features to help you turbo-boost your affiliate marketing business.
Prime Affiliate Links comes with one-click sharing functionality for all your created links, allowing you to easily share your affiliate links across multiple social networks.
LINK EXAMPLES
Here are some examples of branded links created with Prime Affiliate Links.
http://mailchimp.com?ref=8bbdde3a8e688e4c1e4fe03f96f3e466 => http://monowp.com/mailchimp
http://amazon.com?ref=8115495ab1c6166b69f8773cd43c29ca => http://monowp.com/amazon
FEATURES
<ul>
<li>Gives you the ability to create clean, simple URLs on your website that redirect to any other URL (allows for 301, 302, and 307 redirects only)</li>
<li>Basic link tracking and stats (7 days)</li>
<li>Select Temporary (302 or 307) or Permanent (301) redirection for your pretty links</li>
<li>Creates nofollow/noindex links</li>
<li>Manage your affiliate links with Link Groups and Tags – PREMIUM Feature</li>
<li>Share any of your affiliate links with 1 click across all social media – PREMIUM Feature</li>
<li>Automatically replace selected keywords in your site\’s content with affiliate links – PREMIUM Feature</li>
<li>Protect Links with a password – PREMIUM Feature</li>
<li>Add Affiliate banner widgets to your site – PREMIUM Feature</li>
<li>Advanced link tracking features (7dyas, 14days, 30days) – PREMIUM Feature</li>
</ul>

Prime Affiliate Links – Super-Powered Affiliate Links Manager

Prime Affiliate Links is an easy-to-use plugin that helps you sky-rocket your affiliate marketing business by providing a link management solution tha …

Base64 Shortlinks vs Prime Affiliate Links – Super-Powered Affiliate Links Manager