How can I create a menu like this in flex 3.

How can I create a menu like this in flex 3.

This is how I would probably do it but keep in mind there are other ways to accomplish this.
Create a custom MXML component that contains :
The component is data-driven. Here is an example of a xml you could pass to the component (based on the picture) :
<TopItem label="home" />
<TopItem label="envelopes" />
<TopItem label="paper">
<SubItem label="paper and more">
<Leaf label="8 1/2 x 11 Paper"/>
<Leaf label="Notecards"/>
</SubItem>
...
The submenu is added to the stage via the PopupManager so it's always on top of the application.
Tricky parts will be: