Tim Huegdon and Mark Aidan Thomas create an elegant wrapper to the YUI Connection Manager that manages repeating Ajax JSON requests. Tim describes the use of Custom Events, and the use of the Observer design pattern, dissects the code and offers a tidy demonstration of the working code. This simplifies JSON requests into declaring a simple function to process the returned JSON object, and a single line to fire off the request (within a setInterval if required)
A useful overview / introduction of the jQuery library, covering using selectors and XPath expressions to target specific elements in a document, simple Ajax requests, the basic animations available, restructuring a document, creating jQuery plugins.