Download OnyX.
It's a free utility that does system maintenance, and I highly recommend it anyway for keeping your Mac running well.
However, it also has a number of extra functions, one of which is the ability to edit the dock. At the bottom of the dock preference pane within OnyX you can add separators. These will put gaps between certain apps by inserting an invisible icon.
If you want an actual line between the groups you'll need a more comprehensive dock editing program such as SuperDocker.