gentlesource

This plugin creates a short URL from the blog post permalink and stores it 
in the database. The URL is displayed below the blog post along with a link 
to twitter that passes the short URL on.
You can use all short URL services that offer an API. Here are a few:
<ul>
<li>http://melt.li/</li>
<li>http://lin.io/</li>
<li>http://unrelo.com/</li>
<li>http://bit.ly/</li>
<li>http://u.nu/</li>
<li>http://tinyurl.com/</li>
</ul>

GentleSource Short URL

Automatically shortens the blog post URL.

Julio Potier

AutoShortener can create short links in your WordPress website for your posts and pages but also for external links. 
You got many many great options, a usefull admin panel, bookmarklets, shorcodes. 
But easy to use ! Visit website www.autoshort.com for more informations.

BAW AutoShortener

AutoShortener can create short links in your WordPress website for your posts and pages but also for external links.

GentleSource Short URL vs BAW AutoShortener