XMLBlueprint8

XMLBlueprint8 is a commercial tool for Windows based systems, that adds a lot of comfort to XML and DTD editing. English, German, French, Spanish and Dutch languages are supported.

A 10 day test is free; a licensed version costs about $80. 

Like notepad++ features as XML formatting, syntax coloring, XML folding and unicode are supported.
Additional features that will be of interest for those working often directly with XML or DTD files are:

• Context Sensitive XML Completion ('Code Completion')
  The extension specific DTD file can be learned/imported (in the case of mz3 the megazine2.dtd) and the possible options for the actual position in the structure are proposed. A very fast and save way to add elements and attributes (see video on the right) 
• Validation
  With one keystroke the actual XML is validated offline and directly against the valid DTD file.
  And you immediately can modify the incorrect lines 
• FTP and WebDAV
  XML Schemas stored on a remote FTP server or WebDAV server can be edited directly

Those who only modify megazine.mz3 templates from time to time with little changes, this editor will be an overkill. Notepad++ in combination with Validome as free tools will do the job.

But if you get used to build and modify mz3 files more often, you should have a look at XMLBlueprint.