Puppet (http://www.puppetlabs.com/)'s a great tool for system automation - out of the box it comes with most of the functionality that you'll ever need, and a DSL that's easy enough to let junior sysadmins and developers maintain their own recipes. But, depending on the kind of workhorse you want puppet to be, it's possible to hit a wall. For my company, we had too many developers, many system administrators, multiple releases in production simultaneously, and a change control to rule them all (but managed nothing). We also had the odd typo in node definitions causing incidents.
One possible answer to this is to employ an External Node Classifier, which in simple terms means an external script that Puppet calls to get the node definition, or at least the classes and parameters (and the environment, but don't rely on that due to an outstanding bug). An ENC cannot call definitions or puppet functions directly however - it can just pass in classes, class parameters, and global parameters. It's therefore up to you to make your ENC more intelligent.
I think Dan Bode of Puppet Labs gave me the idea of using YAML at first, over a year ago, and I whipped up a basic version in a couple of hours.
Over my long weekend, I decided to write a new ENC library from scratch that would provide the basics that one might want in an ENC, but allows it to be extended with plugins. You instantiate it for a node with some seed data, pass in a loader object, and then include one or more input/override files. It will take care of class, parameter, and substitution-variable overrides, and will spit out a nice Puppet 2.6+ ENC-compatible hash.
The test_puppetenc.rb file is the best source of documentation.
The Code: https://github.com/rmt/puppetenc-ruby