The Yahoo! Web Developer Network provides a one page overview of JSON, giving a quick tutorial on JSON, how to get Yahoo! Web services to emit JSON, offering an output of a JSON object literal as well as using a callback function method. Yahoo! also describes how their web services typically translate their XML structures into JSON.
PPK describes the three main data formats for requesting data from the server, weighing up the advantages and disadvantages. In the ensuing discussion the general feeling is that JSON is the most popular and simplest method.
Douglas Crockford introduces the JavaScript Object Notation, a lightweight data interchange format that's giving XML-based web services a run for its money. Its a subset of the JavaScript programming language, and very easy for programs to parse or generate, and so its not surprising that all major languages now have JSON libraries/code snippets.