When designing the FLVio RESTful HTTP API I ended up choosing XHTML as the data representation format. My natural instinct was to use XML and invent my own schema, but RESTful Web Services convinced me otherwise.

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.