I came across Appcelerator this week (thanks Andy).
Appcelerator is an open source platform that provides everything you need to build rich web, mobile and desktop applications.
From what I saw Appcelerator is a thin layer on the Javascript API making it much easier to program. It also comes with a set of pre-defined effects that can provide some quick pop to your applications.

A great feature is the 'Appcelerator in 5 Minutes' demo. If you need 5 minutes with it to 'get' it, you're a little slow. You can pretty much learn the model in 60 seconds there. The rest is seeing the scope of functionality.

What I found even more interesting is the approach. Appcelerator views the browser as its deployment environment, and interacts with many different backends, including Java, .Net, Ruby, PHP, and more. This separation is an interesting approach and actually matches in concept what James Strachan was talking about in his post 'JAX-RS as the one Java web framework to rule them all?'. Developing web applications consists of two parts, the UI (HTML, JavaScript, CSS, Images), and the backend (Java, .Net, PHP, Ruby, accessed via JSON, REST, XML, etc). Why do we need to build them as one, or with a single technology?

As with anything, there are applications that this approach works well for (Gmail -> GWT takes an all in one approach, but does treat the browser as a 'real' deployment environment), and many it does not. If nothing else, Appcelerator is on my list to spend some more time with.