One of the tools that clearly shows the Pareto Principle - in which 80% of the funcionalities take 20% of the implementation time, and the other 20% take 80% of the time (and are rarelly used) - is Presentation Viewer in Microsoft PowerPoint.
Most people don't know this functionality of powerpoint , which is extreamly usefull. It has been available since 2001 and for sure took a considerable time to implement.
Presentation viewer allows the person presenting to view on his screen more information then the one that is shown to the rest of the audience. This way the presenter can see the slide notes, the next slides, the elapsed time, etc, in order to better manage his presentation.