If you remember Flash MX and previous versions, you'll remember that the primary left-hand toolbar had a two-column layout that was later replaced with a one-column layout in newer versions. What you might not know is that you can switch back to the two-column layout; if you look above the toolbar, you'll see a double arrow against a dark gray background, pointing to the right. If you click the double arrow, the toolbar's layout will shift from one-column to two-column, and the arrow will change to point left.

Click it again, and it'll shift back to one-column.