At Fanlala.com we are working with a contractor to develop a mobile app to work with the site. We are using Services to provide and API for the mobile app to connect to the backend of the site. We needed to provide the developer with documentation of the API. Since the service may change we decided to write a module to provide the API documentation.
This module will list the resources and methods in your service endpoints with the documentation provided in the help section of the services resource given in hook_services_resources() or changed in hook_services_resources_alter().
Different Services servers can provide examples on calling the methods using hook_services_api_create_example_request() and sample responses can be added in hook_services_resources_alter() to be rendered by another hook_services_api_render_response().
I prefer to use the rest_server module with my Services endpoints so my goal is to have complete documentation for the REST implementations. If others want to contribute documentation in the form of examples/sample responses for other servers they can look at servers/rest_server.inc and look at the hooks there or contact me for help.