- [EditX][1] (commercial)
 - [EditX][2] (free version)
 - [oXygen][3] (commercial)
 - [XMLMate][4] (free/open source
   plugin) for [TextMate][5] (shareware)

  [1]: http://www.editix.com/
  [2]: http://free.editix.com/
  [3]: http://www.oxygenxml.com/
  [4]: http://ditchnet.org/xmlmate/
  [5]: http://macromates.com/