When designing API for a web service, the first decision is not about trivial things such as to use JSON or XML, or to use RESTful or SOAP.
API providers should first decide about using API management tools that offers them a package – developer portal, security access control, analytics & reporting, billing, etc
http://www.3scale.net/ is one tool that provides that. At less than 50 requests per day, it is free. Pretty meager, but ya, it is free.
(anway, JSON and RESTful should be the modern way to go)