Steve

This plugin allows you to specify an alternative domain name to use with the built in URL shortening service.
For this to work it requires the chosen domain to be pointed / parked on the hosting account used for your blog.

Custom Shortlink Domain

Customise the domain name used to generate shortlink URLs.

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.

Custom Shortlink Domain vs GentleSource Short URL