You realise that Adobe lets you make any components you want EXCEPT FOR THE PLAYER? Why, because they want to retain control of it. I mean, they publish the spec, people have made players before, and they have been told by Adobe to stop.
They used place that restriction on those accessing the .swf spec, but that restriction has been lifted. I'd have to check, but I think it's been several years now, but it's certainly been more than a year.
The compiler is free and open (http://opensource.adobe.com/wiki/display/flexsdk/Flex+SDK).
That, an editor, and Bob's your uncle.
On Windows, there's also a nice open source IDE (http://flashdevelop.org).
The one thing that's not open source is the player, and that is free.