Whether it's flex, gwt, cappuccino, sproutcore, jquery or mootools, where have you seen extremely exceptional UI on highly sophisticated webapps? I'm talking about interactive, stateful webapps, not the standard list-of-things kind of webapps.
For some reason Gmail came immediately to mind. I know it doesn't seem flashy or amazing these days, but I think it was the first one that really convinced me that web apps were here to stay. I have no idea what they used though, tons of javascript I assume.
<a href="http://280slides.com/" rel="nofollow">http://280slides.com/</a> - Cappucino<p>Not sure I would necessarily use Cappucino myself, but to me, it demonstrated that treating javascript+DOM as a target for a higher level interface can work, and can work well.<p>I guess Outlook Web Access should get an honourable mention - I guess that was the first widely available app. that demonstrated that is was at least possible and was useful.
I'll stick my neck out and say Facebook has an awesome UI. (No idea what was used to make it.) When I first joined Facebook some weeks ago, I was floored by how many features there are and how <i>every single one</i> is usability-optimized for me the newcomer, catching my eye exactly when I need it. The attention to detail is amazing. Sure, it's all just text, pics and videos, but... I'd been making online maps for years, have some fairly advanced stuff under my belt, and still felt like a newbie when I saw Facebook.
<a href="http://dabbledb.com/demo/" rel="nofollow">http://dabbledb.com/demo/</a> wowed me when I first saw it (still does). It was developed using the Seaside framework which has clean support for continuations.
My housemate made <a href="http://www.guestlistapp.com" rel="nofollow">http://www.guestlistapp.com</a> - fairly awesomesuace jquery ui. Snappy fast too.
Google Calendar. I tried to do a google calendar clone in GWT, and I came to admire many features of google calendar, like dynamically selecting a range on the mini calendar and gcal will update the main view.<p>Its so simple and intuitive yet so sophisticated :)
We think we did pretty well on EnStore (www.enstore.com). We built it using Cappuccino, you can check it out here: <a href="http://cappuccino.org/discuss/2009/10/06/cappuccino-in-checkout-3/" rel="nofollow">http://cappuccino.org/discuss/2009/10/06/cappuccino-in-check...</a>
Facebook. The whole experience is so tight.<p>And the chat program/shortcut bar at the bottom is very impressive, and must have been so hard to get right cross-browser.
Seems like most comments here talk about the "intuitive" aspects of the design. Which I agree with -- an app should ideally be intuitive.<p>That said, being intuitive really only is an issue during the user's learning curve. As every vi/emacs user knows, sometimes you can be way more productive once you adapt to the computer, rather than the other way around. Gmail, for instance, has a very poor UI from a traditional perspective, but I find it very easy to be productive once I learned the keyboard shortcuts.<p>Intuitive is generally a good thing, but in and of itself isn't the most critical requirement for being usable, IMO. I prefer clustering of concepts, simplicity of presentation, and workflow optimization much more important.<p>But I still try to design my apps to be intuitive. After all, why not build on common concepts?
Filespots: <a href="http://www.filespots.com/en/index.html" rel="nofollow">http://www.filespots.com/en/index.html</a> - ExtJS<p>Beatiful display of a pure JavaScript application, that not only looks great, but also works great.
I really like the simplicity and 'cue card' approach of Femtoo.com, it also tackles tricky concepts for non-techy users in a non-assuming way.<p><a href="http://www.femtoo.com" rel="nofollow">http://www.femtoo.com</a>
I don't think it matters what it was made in. You can use ANY language, tool, JS, whatever, as long as you are a talented UI designer. I think people tend to think that they can make these great UI's faster and easier by using certain tools, but I don't believe that is the case. YUI may make it faster to create Javascript tables that can be sorted and searched on the client-side, but you still have to learn that system and integrate it into what you have already created. Pick a language, a JS framework, etc, and master it and any UI will be easier and faster to create.
I can't really say whether the ui is good, because I created it, but here you are anyway<p><a href="http://gambolio.com/#/library/" rel="nofollow">http://gambolio.com/#/library/</a><p>The ui and animations were done entirely using Javascript (with help from jQuery), HTML and CSS. The actual games are flash but the ui is all done using standard web technology.
I was really impressed with fluxiom.com (media management app, from the scriptaculous guy I seem to recall) when I first saw it, although I haven't looked at it for a year or two. Beautiful design and lots of attention to detail. The flickr organizer is pretty incredible too.
I have a couple of faves:<p>- MindMeister (brilliant and almost entirely JS+html - not sure which framework though)<p>- SlideRocket (Flex - almost like using KeyNote on the web)
I like FireRift:<p><a href="http://www.firerift.com/" rel="nofollow">http://www.firerift.com/</a><p>Great looking CMS with ajax (oooh! ajax!!) everywhere