Ext JS and MVC
The whole MVC concept moves entirely into the browser. As far as we're concerned, the server can use any technology it wants. It'll generally just provide and consume data to and from the browser. We move back to an MVC implementation that is a little more like the Smalltalk version (different UI elements you see on-screen are views) and each can have their own controller.
Again, this is about breaking down responsibility. Instead of having a single controller take care of an entire page, we can have a search controller, ...