The Puppet OpenStack modules (search puppet in the repository list of http://github.com/stackforge) are probably the largest and most remarkable example of how Puppet is used to manage a complex set of applications that have to be interconnected and configured accordingly.
There are different modules for each OpenStack component (Nova, Glance, Horizon, Cinder, Ceilometer, Keystone, Swift, Quantum/Neutron, and so on); they can be retrieved from https://github.com/stackforge/puppet-<component>; so, for example, Nova's module is https://github.com/stackforge/puppet-nova.
These modules manage all the different configurations via a settings-based approach, with native types that set the single lines of each configuration file (there may be more than one for each component) and with different subclasses that expose all the parameters needed to manage different services or features of each component.