YAML, for the simple reason that it makes for very readable configuration files compared to XML.
XML:
<user id="babooey" on="cpu1">
<firstname>Bob</firstname>
<lastname>Abooey</lastname>
<department>adv</department>
<cell>555-1212</cell>
<address password="xxxx">ahunter@example1.com</address>
<address password="xxxx">babooey@example2.com</address>
</user>
YAML:
babooey:
computer : cpu1
firstname: Bob
lastname: Abooey
cell: 555-1212
addresses:
- address: babooey@example1.com
password: xxxx
- address: babooey@example2.com
password: xxxx
The examples were taken from this page: http://www.kuro5hin.org/story/2004/10/29/14225/062