That's the question

Leopard comes with two kinds of the Dock. A 3D and 2D one. Whichever you choose, 2DOrNot2D is here for you to change it to your liking.

One click - done.
With just one click, you can change your Dock's appearance.

Whenever you wish to change your Dock's appearance, 2DOrNot2D is there for you.
Pricing and availability.
2DOrNot2D is donationware, which means it's free. However, if you like it, we would very much appreciate if you considered making a donation.
2DOrNot2D can be downloaded here, it requires Mac OS X 10.5 or higher (Universal Binary).