A self-hosted short URL plugin for WordPress. See http://labs.andrewhayward.net/wordpress/shortbread/ for more.
shortbread folder into
wp-content/plugins, and activate through the admin interface. Like you do.
Configure Shortbread under
Settings > Short URLs. Yes, it would arguably make more sense to put these settings on the Permalink page, but given that there's been a bug  open about that for four years now, it'll just have to wait.
Shortbread introduces a new function --
has_shortlink() -- which tries to indicate whether a short URL is available in your current context. This allows you to put things like this in your templates.
<?php if (has_shortlink()): ?> <link rel="shortlink" href="<?php echo wp_get_shortlink(); ?>"> <?php endif; ?>
Otherwise, just use
wp_get_shortlink()  or
the_shortlink() as before. Shortbread tries to stay out of the way as much as possible.
As per WordPress's thoughts on the matter, this plugin is released under the GPL .