macosxhints.com - 10.5: Banish the 3D dock from 10.5 →

The 10.5 Dock, as seen at the bottom of the screen, has a 3D effect. If you toss the dock to the side of the screen, however, you get a nice 2D alternative Dock. If you’d like the 2D dock at all times (that is, even at the bottom of the screen), open Terminal and type the following command:

$ defaults write com.apple.dock no-glass -boolean YES; killall Dock

I hate Leopard’s 3D Dock as well. I found this hint very shortly after the 10.5 release and it makes all the difference in the world to me.