You could use the knife toolknife tool.
For a plane along the X Y axis, go to top view NumPad 7. Press K that starts the knife tool. To constrain the new edge to a straight ling press C. Then LMB LMB http://wiki.blender.org/uploads/1/10/Template-LMB.png click on the top edge, move the mouse down and LMB LMB http://wiki.blender.org/uploads/1/10/Template-LMB.png click on the last edge.