The Flex SDK provides a highly productive, open source framework for
building and maintaining expressive web applications that deploy
consistently on all major browsers, desktops and operating systems.

It provides a modern, standards-based language and programming model
that supports common design patterns suitable for developers from many
backgrounds.

Flex applications run in the ubiquitous Adobe Flash Player and Adobe
AIR.

WWW: http://opensource.adobe.com/wiki/display/flexsdk/Flex+4