FAST and cairngorm are vastly popular, but not applicable for every app. As is always the case with frameworks. I’ve used FAST, and seen some cairngorm. Both are nice. Neither is the unanimous only solution. Not to mention, FAST hasn’t been ported to Flex 2 yet, and may not, since Macrodobia owns iteration:Two, makers of cairngorm.
At any rate, check out flexArch. If you’re in need of a custom API to fit your needs maybe it’ll be there, or you can get help in making it. that’s my purpose for being there.