SproutCore has been getting some buzz after the June Apple WWDC, where SproutCore was demoed. This is an MVC framework that builds a standards-based application that runs entirely in the browser and is not tied to a single scripting language to power the back end. The framework is being used to drive the new MobileMe service from Apple.
SproutCore is a framework for building applications in JavaScript with remarkably little amounts of code. It can help you build full “thick” client applications in the web browser that can create and modify data, often completely independent of your web server, communicating with your server via Ajax only when they need to save or load data.
There is a review of SproutCore in Ars Technica, with lengthy discussion about some browser issues that may affect the decision to use it.