Be very wary of SproutCore. It is not only a Rich Internet Application framework. It is also a MVC framework in javascript, which adds a lot of complexity to the design (mostly when compared to GWT and cappucinno).
On the other hand, its designer tool is the only free one among javascript frameworks. However, it is quite new, and a bit buggy.
On the other hand, its designer tool is the only free one among javascript frameworks. However, it is quite new, and a bit buggy.