Interesting... when I did a so-called "Clean Install" of Leopard, I got 3 stacks in my Dock: Applications, Downloads, and Documents. How did you install Leopard?
To the OP: If the Applications folder is NOT in your Dock, you can use
HLdan's method, or press

+Shift+A to open the Applications folder then drag the little icon at the top of the window to the Dock.