YAML may be the most human friendly format for structured data invented so far.
YAML examples and usage in notable Python programs
YAML is a very feature creep language. Most packages below use its subset.
tmuxp - http://tmuxp.readthedocs.org/en/latest/examples.html
ansible - http://www.ansibleworks.com/docs/YAMLSyntax.html and http://www.ansibleworks.com/docs/playbooks.html#id9
appengine - https://developers.google.com/appengine/docs/python/config/appconfig and http://code.google.com/p/rietveld/source/browse/app.yaml
Full YAML parsers
PyYAML - http://pyyaml.org/ - due to complexity of YAML spec, this is the only package that tried to be compliant. It is C-based.
Parsers for YAML subsets
Luckily, you may not need the full YAML spec. These libraries can parse only the sweetest parts of YAML spec that you may reuse in your configs.