So you’re tasked with creating a new web site or enhancing an old one and you need to provide some nice UI components or a RIA, the question is, what to use? We have faced this question at the Digital Libraries, and found the best approach for us is to define the requirements and audience of the application.
Usually, when asking around about js frameworks, the default answer is often jquery and in most cases jquery is a very good choice, but it also depends on what type of application you are building.
A nice comparison of several js frameworks is available here:
If you are looking to build an app-like web project for mobile and tablet devices, the Sencha Touch library looks to fit that niche.