Firstobject's free XML editor for Windows is called foxe is a great tool.
Open or paste your XML into it and press F8 to indent (you may need to set the number of indent spaces as it may default to 0).
It looks simple, however it contains a custom written XML parser written in C++ that allows it to work efficiently with very large XML files easily (unlike some expensive "espionage" related tools I've used).
From the product page:
The full Visual C++ source code for this firstobject XML editor
(including the CDataEdit gigabyte edit control MFC component) is
available as part of the Advanced CMarkup Developer License. It allows
developers to implement custom XML handling functions such as
validation, transformation, beautify, and reporting for their own
purposes.
