Add space to the end on a widget

In your style.css add this:

.widget { margin-bottom: 20px; }

That should give all your widgets 20 pixels of spacing but only works when the widgets are stacked on top of eachother.

If you have multiple sidebars and only want this to happen on a single sidebar, add a more-specific selector i.e.

.my-special-sidebar .widget { margin-bottom: 20px; }

But that requires that you wrap your dynamic_sidebar call in a container with a class of my-special-sidebar.