I believe that Notepad++ has this feature.
Edit (for newer versions)
Install the "XML Tools" plugin (Menu Plugins, Plugin Manager)
Then run: Menu Plugins, Xml Tools, Pretty Print (XML only - with line breaks)
Original answer (for older versions of Notepad++)
Notepad++ menu: TextFX -> HTML Tidy -> Tidy: Reindent XML
This feature however wraps XMLs and that makes it look 'unclean'.
To have no wrap,
- open
C:\Program Files\Notepad++\plugins\Config\tidy\TIDYCFG.INI,
- find the entry
[Tidy: Reindent XML] and add wrap:0 so that it looks like this:
[Tidy: Reindent XML]
input-xml: yes
indent:yes
wrap:0
