Flex is a gigantic library built in pure AS3 by Adobe. They also made a FlexBuilder that uses an xml-based language to communicate and use their own controls. Its pretty amazing what they did - only problem is the second you include the flex library - it's 200K... so developers are trying to stay away from it until Flex is "Bundled" directly in the next flash player.
I personally have my own library and framework i've spent half a year building that I use on every project... she's at about 30K but its just how I want it
