To further customize the look of your Dock in Leopard you can add spacers to the Dock. To add the spacers, you need to open a terminal window (in applications -> utilities) and run one of the following commands:

defaults write com.apple.dock persistent-apps -array-add '{tile-data={}; tile-type="spacer-tile";}'

defaults write com.apple.dock persistent-others -array-add '{tile-data={}; tile-type="spacer-tile";}'

The first command adds spacers to the applications side of the Dock (the left or upper side depending on your orientation) and the second command adds spacers to the other side of the dock. Each time you run the command a new spacer will be added... but you won't see them right away. You need to restart the dock for them to show up. To restart the Dock run this command in the terminal:

killall Dock

The dock will then restart itself automatically and the spacers will then be shown. You can click and drag them to the location where you want them in the dock. You can also remove them by click and dragging them off the dock.