MarcusPope

A WordPress plugin that converts all URL formats to root-relative URLs to enable seamless transitioning between staging/production host environments and debugging/testing from mobile devices, without the use of hackish tactics like textual find-replace strategies or risky hosts/NAT spoofing strategies.
With Root Relative URLs you can browse your development site from http://localhost/ or http://127.0.0.1/ or from a named network resource like http://mycomputername/ without worrying about links redirecting you back to your site’s URL.
This plugin also modifies the tinyMCE hooks so links and media embedded with built-in tools will only insert URLs from the first forward slash after the domain (i.e. the root of your site.) This means when you push content changes to a staging or production environment they are guaranteed to reference the correct target instead of accidentally referencing a production resource in development or, worse-yet, a development-exclusive resource in production.
It supports path-based MU Installations, but does not support domain-based MU sites due to architectural deficiencies in the WordPress core.
Version 1.5 fixes an infinite redirect problem that is a result of a core bug in WordPress. If you have problems with the <!–more–> tag or permalinks for custom post types, please read the FAQ or new Install Steps for support.
Version 2.2 allows for adding certain URL’s or partial URL’s to a blacklist, meaning I won’t use root relative urls, but dynamic absolute URLs instead for displaying content. This will fix problems with 3rd party plugins, and can be configured on the General Settings page.
<h3>Arbitrary section</h3>

Root Relative URLs

Converts all URLs to root-relative URLs for hosting the same site on multiple IPs, easier production migration and better mobile device testing.

Grégory Viguier

WordPress 3.5 removes the setting fields to change the media upload path and url. This plugin enables them again. Note that as long as your fields are not empty, you can disable this plugin.

Upload Url and Path Enabler

Get the media upload path and url setting fields back in WP 3.5+.

Root Relative URLs vs Upload Url and Path Enabler