Now Adobe has released the latest 4.6 version of the free Flex SDK and its commercial IDE Flash Builder. This will likely be the last version of the Flex SDK to be released under Adobe governance, as the SDK will now be transitioning to Apache - like Adobe did with PhoneGap - for further development.
While the Flex SDK has been open source since Flex 3, released in 2007, its development was entirely in the hands of Adobe, with perhaps some inputs from the community. After the release of Flex 4.6 however, further development of the Flex SDK will be in the hands of the community. Developers from Adobe will continue to contribute to the Flex SDK as usual; however non-Adobe developers will also be able to contribute code to the SDK and decide its roadmap.
The Flex 4.6 SDK itself comes with a number of new features, and support for the latest Flash Player 11 and AIR 3 runtimes. First and foremost is a new set of Spark components for mobile applications. Secondly, there is a huge boost in performance; repackaging old applications with Flex SDK 4.6 should give a 50% boost, while new applications should see near-native performance. Finally, the new SDK also brings support for the latest iOS 5 and Android platforms.