As a site which has many nifty JavaScript-driven features, Flickr makes good use of the Yahoo! User Interface library for much of its JavaScript DOM, Event handling and Ajax functionality.
One of the fancier widgets we’ve implemented is a flashy browser-based Web Uploadr which uses the YUI Uploader component (a combination of JavaScript and Flash) which allows for faster batch uploads, progress reporting, a nicer UI and overall improved user experience.
Head over to the YUI Blog and check out how Flickr uses YUI Uploader to provide a faster, shinier upload experience.