jQuery UI: If you're building a "web 2.0" type web application, jQuery UI is a good fit in my experience. It is lightweight for a UI library and if you're alre

jQuery UI: If you're building a "web 2.0" type web application, jQuery UI is a good fit in my experience. It is lightweight for a UI library and if you're already familiar with jQuery you'll feel right at home. I'm working on such an application right now. I have to say that the documentation explaining how to create your own widgets is sorely lacking. I hope to write something soon on this subject. Ext JS: If you're building a monster web app for intranet / corporate use with a 'desktop application' type of GUI rather than a web friendly interface, Ext JS is an extremely powerful solution. It has ridiculously powerful widgets (the Grid for example). Everything is event driven and class based. Making new widgets or extending core ones is easy through inheritance. I've worked on 3 such applications so far and it's been great. The documentation is top notch and it's actively supported. It is however VERY heavy. Both in terms of size and in terms of the markup it generates. Like I said, not for lightweight apps. Build your application completely in JavaScript and forget about html with this one.