UPDATE - A comprehensive comparison, updated as of February 2015, can be found here:
Alternatives to Ext JS
2008 question:
There are a number of great and not so-great Javascript GUI frameworks out there. I've looked at some (only superficially). And I can't make my mind about any of them
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Ext.js The obvious choice by many since it's one of the most known frameworks.
Advantages: Looks awesome, large community, lots of extensions/plugins, GPL'ed
Disadvanatges: Inability to use third-party extensions with commercial license (and some of those extensions have killer features)Backbase Relatively less known. A curious mix of XML and Javascript that is reminiscent of XUL. However, it's already cross-browser
Advantages: Looks good, very extensible, allows easy incorporation of some really neat stuff
Disadvantages: Pricing is steep and CPU-bound (though free to use on up to 2 CPUs), forums are slow to respond (though commercial support is supposedly fast)qooxdoo Also very popular.
Advantages: Please, fill in
Disadvantages: Code is slighly messy (based on hearsay)YUI Fill in description