ExtJS is an open source JavaScript library for building rich internet applications. ExtJS if a fully object-oriented framework that is extensive in its capabilities. I have even gone as far as creating a desktop like web portal.
ExtJS does have a learning curve to it. Once you get it then just about anything can be done with this framework. If your application involves a great deal of AJAX functionality then ExtJS is the best choice. If fits well with making the data requests and also storing the results as data for us on the page.
My work with ExtJS involves true object-oriented client side programming. Especially important in this area is build composable and reusable objects so that new objects can be created easily.