Make some space
Sometimes little details gives a way greater feeling of power and customizability, without actually adding much and more importnant without adding tons of new buttons and configuration options.
Plasma panel can arrange widgets in an ordered linear layout, one after another, but what about if you have a panel where you want to distinguish between different groups of applets and want to put a space between them to make the look more clear? Since a couple of days in the plasma configuration dialog a new single button appeared: "Add spacer":
Clicking on it it adds that little widget at the end of the panel, it's just a space and you can drag it around as with every other widget or resize it as you wish by dragging the borders of it (that in this screenshot are a little darker) just like any window or resizable stuff in general.
Of course a space is not something that you should see: it's empty right? So you close the panel edit mode and the empty space ecomes well, empty, until you open again the panel configuration mode.