As I’m sure you have read by now, we have announced that Flex is going open source.
One cool part is that not only is the Flex framework – meaning all the ActionScript source files – open, but the Flex compiler (mxmlc and asc) and command-line debugger (fdb) are also open. Yes, the source of those will be available for anyone to modify and enhance, under the same terms as the framework (the Mozilla Public License).