xmllint from libxml2

xmllint --format file.xml

(On Debian-based distributions, install the libxml2-utils package.)

xml_pp from the XML::Twig perl module

xml_pp < file.xml

(On Debian-based distributions, install the xml-twig-tools package.)

XMLStarlet

xmlstarlet format --indent-tab file.xml

Tidy

tidy -xml -i -q file.xml

Python's xml.dom.minidom

echo '<root><foo a="b">lorem</foo><bar value="ipsum" /></root>' |
  python -c 'import sys, xml.dom.minidom; print(xml.dom.minidom.parseString(sys.stdin.read()).toprettyxml())'

saxon-lint (my own project)

saxon-lint --indent --xpath '/' file.xml

saxon-HE

echo '<root><foo a="b">lorem</foo><bar value="ipsum" /></root>' |
  java -cp /usr/share/java/saxon/saxon9he.jar net.sf.saxon.Query \
       -s:- -qs:/ '!indent=yes'

xidel

xidel --output-node-format=xml --output-node-indent -se . -s file.xml

(Credit to Reino)

Output for all commands:

<root>
  <foo a="b">lorem</foo>
  <bar value="ipsum"/>
</root>