Did you ever get a stream of XML out of a log file, or in a data stream, and it's all mashed together without line-breaks so that it just appears as gobble-de-gook? If there's a data error (not an XML parsing error) then you have to read it so that you can find where the error is, but you don't have XML-spy and NetBeans is overkill or takes forever to fire up...

Emacs to the rescue! Benjamin Ferrari wrote this increadibly useful (and simple) elisp function to pretty-print a block of XML code:

(defun bf-pretty-print-xml-region (begin end)
  "Pretty format XML markup in region. You need to have nxml-mode
http://www.emacswiki.org/cgi-bin/wiki/NxmlMode installed to do
this.  The function inserts linebreaks to separate tags that have
nothing but whitespace between them.  It then indents the markup
by using nxml's indentation rules."
  (interactive "r")
  (save-excursion
      (nxml-mode)
      (goto-char begin)
      (while (search-forward-regexp "\>[ \\t]*\<" nil t)
        (backward-char) (insert "\n"))
      (indent-region begin end))
    (message "Ah, much better!"))

Indeed, why isn't this included with NXML-mode? Ah well, at least my problem's solved for the simple use-case of opening a machine-generated XML file and wanting to visually parse it...

Benjamin's blog has lots of useful Emacs stuff. Worth a read.