Unlike editors for support of DTD or XMLSchema, editors which support RelaxNG are yet quite scarce. But this will change over time, especially once that DocBook release 5 will get out.
Please note that on this page we are speaking only of those editors we actually use on a daily basis.
Visual editing is very helpful on during data-input. Most solutions use CSS for delivering the GUI.
The XMLmind XML Editor Professional Edition is a commercial editor.
A personal version, the XMLmind XML Editor Standard Edition, can be used at no cost.
Standard or Professional, both versions have outstanding good RelaxNG support and are highly customizable. Java-based, they run on every platform.
Text editors instead are good for the developers, or for a quick hack.
The oXygen editor is Java-based, and should have everything you need.
The Exchanger XML Professional Edition is an editor very much like oXygen.
Lately, there is also a light version for personal use only, the Exchanger XML Lite.
The Eclipse IDE together with the XMLBuddy Pro plugin gives a thorough development environment.