Depends on how complex the application is. -JQuery UI for simple widgets to make the UI a bit more functional with JS - ExtJS for when the whole web app is a

Depends on how complex the application is. -JQuery UI for simple widgets to make the UI a bit more functional with JS - ExtJS for when the whole web app is a JS application with rich, desktop-like interactivity. Use mostly JSON for communication with the ASP.Net MVC backend. - YUI for anything in between those two points mentioned above. I haven't really tried anything else so far. But I'd like to look at GWT when I can.