Adobe Flex is powered with all major three areas of development : Desktop (RIA), Web & Mobile.
Adobe Flex SDK is an open source project since Feb 2008.
Adobe Systems provide an IDE Flash Builder (formerly known as Flex Builder) for development. It’s available with trial (60 days) and licensed only.
Adobe Flex Applications are developed using 3 programming languages (MXML- , Actionscript and FXG- Flash XML Graphics).