The Dock is so nice, but one of the things that it doesn't do well is let you really customize it.  This hack lets you organize your dock in a little bit different of a manner.  By writing to your defaults you can add in an empty space to your applications or folder side of the dock.  Why would one want this?  Well it works to your advantage when you try to group your applications in the Dock.  By having all of your audio-editing or video editing icons in one spot, or having your Creative Suite all together is nice.  It also lets you see the background of your dock (if you've modded that).  It also allows you to see clearly how many non-persistent applications you have open (non-persistent apps are ones that do not stay in the dock).

The command for the applications spacer is
defaults write com.apple.dock persistent-apps -array-add '{tile-data={}; tile-type="spacer-tile";}'

The command for the documents spacer is
defaults write com.apple.dock persistent-others -array-add '{tile-data={}; tile-type="spacer-tile";}

This mod comes to us via Mac OS X Hints UK site.