Putting a folder in the dock
You can't put a folder in the left side of your Dock where the programs are, but you can drag a folder to the right side of your Dock. In Leopard (10.5) or later, by default, the folder will appear as a stack, but starting in 10.5.2, you have options for how the folder or stack appears. In Lion (10.7), for instance, you can select whether you want the folder to appear as a folder, or as a stack.