If you want to divide the applications in your Dock into groups, the best way is to add spacers. A common method is to create “fake” applications with an invisible icon to fill the space, but that is a bit inelegant. A much better way, available in Leopard, is to use a Terminal Command. Each time you run the command it inserts a blank "space" into your Dock, which you can then drag around to where you like.

Start by opening up Terminal, located in Applications/Utilities. Type the following line and then hit enter:

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

For the changes to take effect you will have to restart the Dock, either by logging out and in again or by running the killall Dock command in the Terminal. The spacer appears at the end of the icons that are permanently in the Dock. You can then drag it along to where you want it.

The above command inserts the spacer on the applications side of the Dock. To insert a spacer on the right hand side, use the following command:

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

As always, you will have to run killall Dock for changes to take effect. To add multiple spacers just run the command again and again. Note that you can use the “up” arrow in terminal to automatically enter the last run command.

If you want to remove a spacer, treat it like any other icon in the Dock. Either drag it off and let go to get a nice puff of smoke, or right-click on it and choose Remove from Dock.