What is the cleverest UI feature you have seen in a website? [closed]

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ColorSchemeDesigner

we can have a page preview by selecting a color scheme which helped me a lot in designing web pages for my applications.

I've seen something completely against all standards and it's also more an experiment, but it definitely has its wow factor. It's called Don't Click It and it's all about using an UI without clicking. Try it out for yourself, you'll be surprised how often you're tempted to click (at least that was my experience)

Made the user experience more intuitive

Support for the back button and tabbed browsing, which GMail is a prime example.

I've used far too many ( Oracle based time booking systems seem the worst for this ) which just screw up if you use 'back' to go back to the page you were on before you clicked on a link.

On one site, nationalrail.co.uk, it's almost impossible to get train times for two different queries open in different tabs, as it stores the query on a per-user basis. ( so if you open one tab, make a journey query, open another tab, make another query, then ask for later trains on the first tab you get the details for the second journey ). Sometimes I have to open chrome and firefox or use more than one computer to plan my journey.

Google maps is by far the most clever/advanced non-flash UI on the internet.

Drop-down suggestions for a search box, like ebay, amazon, etc. all use on their sites to make it easier for users to find search keywords. Google has this too, but the feature is arguably even more useful on a site-specific search, since you don't get random keywords which don't apply to that site.

I was involved with getting the search suggestions feature onto MSDN Search several years ago, and the usage metrics since then have been phenomenal: as many as 20% of English searches use the auto-suggest feature. In search, a feature is typically successful if over 3% of searches use it, since almost all the time when searching, people just choose the default UI and move on. 20% usage is unheard of-- no other search feature I've seen (on any site) comes close.

What's particularly nice about search suggestions is that they only take up screen real estate when someone actually needs them (typing into search box) and take up zero page real estate otherwise.

Also, you can extend them beyond simple suggestion lists-- you can show preview UI (like IE8's browser-toolbar-search-box's preview images do, or even allow one-click links to search results if users want to bypass search altogether).