Note: For some reason completely beyond me Google Sites does not allow applets anymore. I relocated the applet and copied the page here -

The following applet demonstrates Voronoi diagrams.
  • Click adds, right click deletes, dragging moves around points on the yellow plane below.
  • The applet automatically¬†computes Voronoi cells, which enclose the closest points on the plane for each of the points that you place.