A simple tool that gives you the option to measure distances.
From the place where you clicked a line is drawn with a number showing the distance to your mouse cursor, until you release the mouse.
By default this is publicly visible for all players in the session, but you can change the toggle to make it private.
The colour in which this is drawn can be chosen by each player in their client settings.
The ruler also has snapping behaviour that will try to snap to logical places on the grid.