I needed some JSON classes for my mqweb project. I know there are some classes in the sandbox of Poco for JSON but I couldn't use these classes. So I developed my own classes. I've used the Coco/R parser generator to generate the JSON parser. Coco/R is tweaked to make it look like a Poco application and to make it generate classes that use Poco. There is still some work to do, but the classes can be used from now on (still need to add some features like unicode for example).
You can find the JSON classes and the tweaked Coco/R here.