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.

Pierre-André Boissinot

Semio Design

When you use the WordPress editor to insert a media in the content field it stores a reference to the absolute path. 
So if you change the location of your site (by switching from staging to production environment for example) all your paths will be incorrect. 
This plugin stores relative pathes in the db instead of an absolute path. 
Beware: this plugin must be activated BEFORE inserting a media into the content field. It has no effect on the medias stored before the activation. 
It’s mostly based on the WP_SITEURL constant which should be defined in the wp_config.php file. If this constant isn’t defined it uses the “siteurl” option value instead. 
If the plugin detects that you access the website from another location than the “siteurl” defined initially it can correct most of the wrong urls in the database.
Localisation of the plugin into french.

Simple Relative Content

This plugin makes all attachments urls stored in database relative to the WP_SITEURL url.

Relative Image URLs vs Simple Relative Content