Relative Site URL on Content Save

Allows you to publish content during dev phase without having to go back and
fix your content’s static URLs when you switch to the production URL. Registers [url] shortcode
with your site’s base URL setting. Replaces occurances of http://mysite.com/the/link
with [url]/the/link inside the content editor when you hit Publish or Update.

http://chrisbratlien.com/2010/05/16/relative-site-url-on-content-save/