Adobe announced their plan to open source Flex, I'm very excited about this news! Just got back from Toronto today so just blogging some resources to check out, will certainly blog more about this move when I've had a chance to review it all.
Adobe is announcing plans to open source Flex under the Mozilla Public License (MPL). This includes not only the source to the ActionScript components from the Flex SDK, which have been available in source code form with the SDK since Flex 2 was released, but also includes the Java source code for the ActionScript and MXML compilers, the ActionScript debugger and the core ActionScript libraries from the SDK. The Flex SDK includes all of the components needed to create Flex applications that run in any browser - on Mac OS X, Windows, and Linux and on now on the desktop using “Apollo”.
Developers can use the Flex SDK to freely develop and deploy Flex applications using either Adobe Flex Builder or an IDE of their choice.
The source code for the Flex framework is already available within the free distribution of the current Flex 2 SDK. By this summer, Adobe plans to put in place most of the infrastructure (public bug database and public daily builds) required to run the Flex SDK as an open source project. We expect to complete the transition to a fully open source project (source code for the compiler, infrastructure for community contributions, etc.) by the end of 2007.
Robert Scoble has some great video interviews with the Flex team about going open source.