Override the default WordPress "shortlink" URL with one that has a custom text in it. You can also set a different base URL.

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RevCanonical

A Wordpress plugin that creates and adds support for shortened urls plus the shortlink auto-discovery tag

akrabat

The Shorter Links WordPress plugin overrides the default WordPress 
“shortlink” URL with one that has a custom text in it. You can also set a 
different base URL.
A custom field called “Shorter link” is created once a post is saved, 
so that you can change the shortlink to a more memorable set of 
characters.
The choice of base URL to use for the short link can be configured within 
Settings->Shorter Links.
Related Links:
<ul>
<li><a href="http://akrabat.com/shorter-links" rel="nofollow ugc">Plugin home page</a></li>
</ul>
<h3>Licence</h3>
This plugin is licensed under the <a href="http://akrabat.com/license/new-bsd" rel="nofollow ugc">New BSD license</a>.
<h3>History</h3>
2.1.0 – 8 August 2015 
Rework to look for the shorter link after WordPress has done its processing. This 
means that a shorterlink that represents a date won’t affect an archive list.
2.0.6 – 8 August 2015 
Ensure that the short_link is correct when using a post id.
2.0.5 – 8 August 2015 
Updated Tested up to 4.3
2.0.3 – 9 July 2012 
Bug fix so that archives work.
2.0.2 – 23 June 2012 
Fall back to REQUEST_URI if there’s nothing interesting in $query_vars.
2.0.1 – 20 June 2012 
Updated to handle 4 digit short links that look like a year to WordPress.
2.0.0 – 21 November 2010 
Updated to be WordPress 3.0 or above, so we only need to hook into the WordPress 
shortlink system
1.8.2 – 21 November 2010 
Fix permissions issue on settings page. This is the last version that works 
on WordPress 2.9.x or earlier.
1.8.1 – 7 September 2010 
Bug fix to remove a warning.
1.8 – 1 September 2010 
use shortlink rather than shorturl for WordPress less than 3. 
For WordPress 3 or higher, hook into the new shortlink system. 
Fix the admin page so that it displays in WordPress 3.
1.7 – 11 Feburary 2010 
Handle failures better.
1.6 – 10 January 2010 
Update version number in correct places so that the WP plugins system notices the update.
1.5 – 29 December 2009 
Support permalinks that start with /%category%/
1.4 – 14 April 2009 
Support rel=”shorturl” as per <a href="http://sites.google.com/a/snaplog.com/wiki/short_url" rel="nofollow ugc">Robert Spychala’s Short URL Auto-Discovery proposal</a>.
1.3 – 14 April 2009 
Add support for setting the base URL. Patch by <a href="davemastergeneral@gmail.com" rel="nofollow ugc">Dave Marshall</a>.
1.2 – 13 April 2009 
Only send the <code>Link</code> HTTP header as recommended by <a href="http://shiflett.org/blog/2009/apr/a-rev-canonical-http-header" rel="nofollow ugc">Shiflett</a>.
1.1 – 13 April 2009 
Fixed output of HTTP headers. Patch by <a href="http://bethesignal.org/" rel="nofollow ugc">Jeff Waugh</a>.
1.0 – 11 April 2009 
Initial release.

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