Oxygen XML

From Aventine Solutions

A commercial XML editor I recommend. It has a stand-alone version and a feature rich Eclipse plug-in. It is implemented with Java and thus works equally well on MacOSX, Linux and Windows (I have used it myself on all of these platforms). For Eclipse users, it is recommended when advanced XML editing, XSLT (it has a great debugger for this) transformation and SOAP/WSDL web service development support is needed and the editor that comes with Eclipse just doesn't cut it. It is reasonally priced, stable and frequently enhanced.

[edit] http://www.oxygenxml.com

A worthy competitor, but only in the Windows arena, is Altova XML Spy.