We've been finding ourselves building more and more JS heavy apps here at &yet. Until recently, we've pretty much invented a custom app architecture for each one.
Not surprisingly, we're finding ourselves solving similar problems repeatedly.
On the server side, we use django to give us an MVC structure to follow. But there's no obvious structure to your client-side code. There are some larger libraries that give you this, but usually have a ton of widgets etc. I'm talking about solutions like Sproutcore, YUI, or Google Closure and there are toolkits like GWT and ...