Relative Links for WordPress
A relative link is a link to a page or a file which is located on the same site the page containing the link is on. *gasp* When activated, this plugin filters WordPress' output, removing redundant stuff from on-site links.
How to use it
Download the latest version of relative-links.php, drop the file in the wp-content/plugins/ directory, activate the plugin, and you're all set.
To speed things up a bit, you may replace the line that says $url = get_root(get_settings('siteurl')); with $url = 'http://yoursite.com'; (the untrailingslashed root URI of your site).