Kind of what Firebug does for HTML/Ajax applications but for Flex 2.0/3.0 applications.

Check it out a live at: http://www.mieuxcoder.com/data/2007/12/FlexSpy-1.2/dashboard.html (in the application, click the flexSpy button on the top-right corner)

Flex-Spy allow you to inspect and dynamically change most properties and styles of the visual components in your Flex application. You can - for example - try out a specific width or a background color for your component before changing it in your code.

To install it in your application, follow the InstallationInstructions.