|Flex is a free open source framework that helps you build dynamic, interactive rich internet applications.
Flex applications are delivered on the web via the Flash Player or to the desktop via Adobe AIR.
Flex applications can be built using the free Flex SDK, or the commercial Adobe Flex Builder 3 an Eclipse based IDE software to accelerate development. The Flex Builder 3 can be tried for free for 60 days.
More information about Flex:
Adobe Flex Getting Started Guide
Adobe Flex Developer Center