Joyent's Smart Platform is an open-source web application development platform (PaaS) that provides cloud administrative services. Developers can upload code and the Smart Platform will setup the infrastructure needed by the website, including: the data store, content store, backups and software upgrades. Server capacity and bandwidth will scale dynamically to meet the demand.
The Smart Platform has an API that provides all the available features of core JavaScript 1.8, which includes E4X, a programming language extension that adds native XML support to JavaScript.
For more background on the API: http://smart.joyent.com/docs/