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.
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
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.
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.