Sometimes I need to look inside XML files which consist of one big line. It's very impossible, and reformatting takes a time. Fortunately, I've found a good solution for vim.
I added to my ".vimrc" file:
map <F2> <Esc>:1,$!xmllint --format -<CR>
Now I can press "F2" button, and vim indents the text.
More hints on indenting XML in vim: Tip #349: Format your xml document using xmllint.