While explaining to a customer today about simply using a web browser to help debug the API I said,
"It is no coincidence that we use XHTML to represent data as it is not only a well-understood XML format but also makes life much easier when debugging."
Which has proven itself true so far. Any browser becomes a debugging tool for the API. Although, until browsers support all the HTTP verbs (or XHTML5 / Web Forms 2.0) you'll need an addon like Poster for Firefox to test commands like PUT and DELETE.