Scott Werner

When inserting an image into a post, WordPress uses absolute URLs to the files. This plugin overrides that functionality and forces it to use relative URLs.
For example:
WordPress Default: 
http://www.example.com/wp-content/uploads/2013/04/example.jpg
Plugin Override: 
/wp-content/uploads/2013/04/example.jpg
This is particularly useful if you plan on switching domains ever as well as reducing HTTP requests.

Relative Image URLs

Creates relative URLs for images when inserting into posts.

Chris Bratlien

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/

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 …

Relative Image URLs vs Relative Site URL on Content Save