Are you looking for any interesting and beautiful Web User Interface libraries for your next web site or Rich Internet Application ? Take a look at this list with ten WUI I suggest you to try in order to simplify your work and develope faster beautiful and usable user interfaces.
1. IT Mill Toolkit
IT Mill Toolkit is an open-source framework, providing widgets and tools for the development of Rich Internet Applications (RIAs). Deliver web applications without worrying about incompatibilities of web browsers, DOM or JavaScript by using standard Java tools.
2. LivePipe UILivePipe UI is a suite of high quality widgets and controls for web 2.0 applications built using the Prototype JavaScript Framework. Each control is well tested, highly extensible, fully documented and degrades gracefully for non JavaScript enabled browsers where possible
3. Iwebkit iPhone/iPod touch frameworkIwebkit is the revolutionnairy kit used to create high quality iPhone and iPod touch websites in a few minutes. In the first 4 months of it's existance the pack has greatly evolved from a basic idea to a project that has reached worldwide fame!
4. JitsuJitsu contains an integrated set of tools to enable developers to build and deploy sophisticated user interfaces for web applications. These include an Xml markup language, page compiler, data binding engine, JavaScript runtime, control library, runtime inspector, animation engine, cross-platform library, Ajax, and back button support.
5. MochaUI MochaUI is a web applications user interface library built on the Mootools JavaScript framework.Uses: web applications, web desktops, web sites, widgets, standalone windows and modal dialogs.
6. Echo Web Framework
Echo is an open-source framework for developing rich web applications. From the developer's perspective, Echo behaves as a user interface toolkit--like Swing or Eclipse SWT. AJAX technology is employed to deliver a user experience to web clients that approaches that of desktop-based applications.
7. The Yahoo! User Interface Library (YUI)The YUI Library is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, DHTML and AJAX. YUI is available under a BSD license and is free for all uses.
8. Sigma Ajax UI builder
Written in javascript and PHP, SigmaVisual is web based visual AJAX UI builder for professional developers. Developers save their time in building up prototype as well as real web applications. TreeBar, TreeGrid, Layout, Menu are supported.
9. WUI Web UI FrameworkWUI (Web User Interface) is an MVC framework for writing web UIs in a single language: Java. Write web apps with components, widgets & events, not JSPs. Runs in any servlet 2.3 container. Similar to Model 2 / Struts, only better. Apache-style license.
10. Butterfly Web UI
Butterfly Web UI is a component oriented web framework for Java, like Wicket or Tapestry. The main advantage compared to these frameworks is that Butterfly Web UI integrates naturally with Butterfly DI Container, giving you a state-of-the-art dependency injection container to help you structure and decouple the internal components of your web applications.
Add a comment to suggest other interesting WUI. Thanks!

Nice one...Thanks for this...
what about extjs?
http://www.extjs.com/
: )
A'' the listed web UIs are really fantastic and inspirational.
thanks for sharing woork!!
very interesting post. i like it.
Jquery examples
http://www.csshook.com
ExtJs is pretty cool too, but I find UI libraries to be an "overkill" for most situations
Another good post! Here are some more examples, I guess:
Prototype
Dojo
Mochikit
Cheers!
extjs [http://extjs.com/] definitely deserves a mention on this list.
yeah... I think JQuery UI should be in this list :)
I was looking at extjs but then found smartclient, which provides the back-end integration that extjs leaves for third-party libraries. Smartclient has everything that extjs has, and is free! They've got some new stuff in their 7.0beta that really rocks!
I agree with Hasan, Jquery UI is so well rounded and finnised, it definatly deserves a place on the list!
Thanks for the Iwebkit iPhone/iPod touch framework link.
Your blog is great!
IT mill tool kit .. is a cool stuff.
It is very cool.... There are any Blogger related UI
iUi seems to be a better fit for iPhone devs.
Thanks for the resource.
Nice list :-).
what about RichFaces?
http://jboss.org/jbossrichfaces/
Demo:
http://livedemo.exadel.com/richfaces-demo/welcome.jsf
One more addition to your list is DHTMLX toolkit, a set of 17 JavaScript/Ajax components, including datagrid, tree, tabbar, windows, etc. Available under both GPL/commercial licenses.
Thanks for the UI Links first of all.
And for everyone else posting links. We all know the "Pay" sites for the UI I think more people are just looking for a Free but Complete UI.
I can't believe that GWT (Google Web Toolkit) was not included in your list. It certainly ranks higher as a true development library since you write the code in Java and it creates the required HTML and JavaScript.
http://code.google.com/webtoolkit/
Thanks for list!
Extjs and GWT should be on the list
Rajesh