OpenLaszlo is an open source platform for the development and delivery of rich Internet applications. It is released under the Open Source Initiative-certified Common Public License
Alloy is a UI metaframework that provides a consistent and simple API for building web applications across all three levels of the browser: structure, style and behavior.
jQuery UI provides abstractions for low-level interaction and animation, advanced effects and high-level, themeable widgets, built on top of the jQuery JavaScript Library, that you can use to build highly interactive web applications.
Let's face it: do you really need drag-and-drop, resizable windows or sortable lists in your web applications? Websites are not desktop applications. They are different.