How to populate one list box after choosing an item from a drop-down list?

Does anyone have the code for populating a list box from a drop-down selection?

What I want to do is give the user a dynamic drop-down list.  (I already have this)
And when they make a selection from the drop-down list, the list box beside it gets populated dynamically from their selection.

My drop-down list will be populated with catagories from my table.
When the user selects one catagory, the list box gets populated with the items from the catagory drop-down.