At Yahoo, I do a lot of UI/Interaction design work. During the design process its helpful in some cases to be able to create interactive prototypes to show working versions of our projects before going into development. One of the tools that I absolutely love and have been using quite a bit lately is Adobe Flash Catalyst CS5. It’s like flash for dummies. Basically you can take a PSD or Illustrator mock and quickly build transitions/interactions into your designs without using any code or ActionScript. See the video below to see it in action.
Presumably it’s not JUST for mock-ups and prototypes?
It will do a working finished piece?
You’re correct, you can produce applications that you can pass on to a Flex developer and they can finish it off using Flex. I haven’t used it for that purpose as I haven’t the need to, but from what I hear, it was developed for that in mind.