?json=1 to the end of any WordPress URL will work like the RSS option, but the feed will be formatted as JSON. It can even be used to submit comments, if you invoke the right method.
This plugin was created for The Museum of Modern Art, whose weblog Inside/Out appears within an existing structure built with Ruby on Rails. Instead of reimplementing the site templates as a WordPress theme, we opted for a Rails front-end that displays content served from a WordPress back-end. JSON API provides the necessary interface for retrieving content and accepting comment submissions.
I like Chris Coyier’s suggestion of using it in conjunction with the jQuery UI Autocomplete component in order to add a Google Suggest-type feature to your search form.
WordPress JSON API Plugin [Digging into WordPress]
JSON API Usage Guide [JSON API]